Mitsuru Sutoh (須藤満, Sutoh Mitsuru in Japanese order, also romanised Suto) was the bassist of T-Square for fourteen years, from the autumn of 1986 to the summer of 2000, and he has gone on playing in its orbit ever since. He was born on 14 December 1964 in Yamagata, the prefectural capital in northern Honshu, and took up folk guitar at 13 and electric bass at 15. At Yamagata Higashi High School he played in amateur bands while also belonging to the school's cheering squad, and he then moved to Tokyo to attend Tokyo Gakugei University, where he joined the light music club.

In 1986, in his third year at university, he heard about an audition. The Square, as the band was then called, needed a bassist to replace Toyoyuki Tanaka, and Sutoh knew someone who knew the drummer Hiroyuki Noritake. He passed the audition and joined formally in November 1986, and his own site still dates the start of his professional career to that autumn. His first album with the band was Truth (CBS/Sony, 1 April 1987), the record whose title track became the theme of Fuji Television's Formula One broadcasts. Discogs credits him on bass beside Masahiro Andoh on guitar, Takeshi Itoh on saxophone and Lyricon, Hirotaka Izumi on keyboards and Noritake on drums. That five-piece is the line-up most people mean when they talk about classic T-Square.

Japanese Wikipedia counts fourteen original albums with him in the band, from Truth to the self-titled T-Square of April 2000. Among them are Yes, No. (1988), Wave (1989), the first album released under the T-Square name, Natural (1990), New-S (1991), Impressive (1992), Human (1993), Welcome to the Rose Garden and Miss You in New York (both 1995), B.C. A.D. (1996), Blue in Red (1997), Gravity (1998) and Sweet & Gentle (1999). When Masato Honda took over the saxophone chair in 1991 Sutoh stayed, and the Sutoh and Noritake rhythm section carried the band through the Honda years and into the Village Records era. His instrument grew with the music. He started on a four-string bass; Discogs credits him on five-string and six-string bass on Impressive (1992) and on five-string bass and six-string fretless bass on Human (1993).

He also wrote for the band. His first composer credit is "Shadow" on Yes, No. (1988), and others follow through the 1990s: "Nab That Chap!!" on New-S (1991), "Sunnyside Cruise" on Welcome to the Rose Garden (1995), "Pioggia di Capri" on B.C. A.D. (1996), "From the Bottom of My Heart" on Blue in Red (1997), "Ms. Bracing" and "Explorer" on Gravity (1998), and "Our Fortress" and "An Evening Glow" on the 2000 album. On Solitude (1994), the T-Square and Friends album dedicated to Ayrton Senna, Discogs credits him as composer and producer of the track "Quiet Moment." Away from the band he is named on Andoh's solo album Melody Go Round (1990) and, in the same year, on a Konami game-music album, Kukeiha Club (1990), alongside Noritake, Honda, Izumi, Akira Jimbo and the bassist Tetsuo Sakurai. Fans also know him as the band's master of ceremonies, a job he took over from Izumi after the early 1990s and has done at its anniversary concerts since.

His own records began in 1997 with Favor of My Friends, a live set with the guitarist Isamu Ohashi, who also produced, Jimbo on drums and Azuchi Takahashi on keyboards. Six of its seven tracks are Sutoh's own, among them "On the Track," "Asian Mogler" and the ten-minute closer "The Sun Is Rising Again," and Ohashi contributed "Success Moon." The Discogs listing carries the Village catalogue number VRFL-0001.

In August 2000 the band dissolved its old format, and Sutoh left along with Noritake, the saxophonist Takahiro Miyazaki and the keyboardist Keiji Matsumoto. A month later he left his management company, Village-A, and went freelance. In 2001 he took a live series also called Favor of My Friends around Japan from January to September, and a session at Live Spot RAG in Kyoto in September 2001 turned out to be the seed of his next band. He became a regular touring bassist for the vocal group The Gospellers from 2001, and he wrote "Rushing Spirits," the official theme song of Montedio Yamagata, the football club of his home prefecture. The single came out on 22 March 2003 in an edition of 1,000 copies, and a live version of it is the sixth track of Favor of My Friends 2003 (Village Records, 4 June 2003), with Keisuke Komori on drums, Hideyuki Yonekawa on guitar and Takahashi on keyboards.

He never really left T-Square behind. For the band's 25th anniversary in 2003 he came back after three years with Noritake and Izumi, and with Andoh and Itoh the group played for a limited period as The Square. Discogs credits him on that year's Spirits, where "The End of the Summer" is his piece, and with a bass solo on T Comes Back. He appears as a guest on Passion Flower (2005), on the 30th-anniversary Super Band project of 2008, on Smile (2013), on the 2018 40th-anniversary reunion concert recordings and on Vento de Felicidade (2023), and on 4 December 2026 he is scheduled to join T-Square's current line-up at the Crossover Japan festival at Tokyo International Forum.

The long-running home for his own writing has been Trix. The band's core is the rhythm section of Sutoh and the drummer Noriaki Kumagai, formerly of Casiopea, and it grew out of the Kyoto session of September 2001. It took the Trix name and began recording in 2004, and Index (King Records, 21 April 2004) was followed by a new album almost every year through Parade (2023), the twentieth, and the 22nd, Chance, arrived in December 2025. Sutoh is credited as composer on most of them, from Index to Ring (2021). The band is known for comic stage business, including wigs worn on stage, on top of very exact playing.

Other long associations include Korenos, a trio with Noritake and the guitarist Hirokuni Korekata that made three albums for Ragmania between 2004 and 2007, and the Hatsuho Furukawa Piano Trio, again with Noritake, which recorded for VEGA Music Entertainment in 2007, 2010 and 2020. In 2006 he joined Honda, Noritake and Matsumoto, all former T-Square members, in Masato Honda with Voice of Elements, whose album appeared on 26 July 2006. Session credits range wider still. He played bass on the vibraphonist Toshihiro Akamatsu's Focus Lights (2005), on Kaori Kobayashi's Solar (2005), on Kazuki Katsuta's first solo album (2014) and on the Dimension albums with Satoshi Bandoh on drums, and on records by the saxophonist Miku Yonezawa, the singer Grace Mahya and the keyboardist Dai Sakakibara (Trans, 2001).

Waking Up, Remember the Day, 2011 (King Records, 7 December 2011) is his widest-ranging solo record. He produced and arranged the album and wrote most of it, and the guests include Noritake and Kumagai on drums, Matsumoto on piano, Miyazaki on EWI and The Gospellers, who sing "Bass'n Voice" and a version of Simon and Garfunkel's "Bridge Over Troubled Water" with him. He plays five-string and six-string bass and, on the opening track, a six-string banjo. A live DVD of the Waking Up tour from Blues Alley Japan in Tokyo followed in 2012.

He also made two duo albums with Izumi under the name Hiromitsu, 6 on 224 (Misty Fountain, 6 July 2014) and Wide & Full (Misty Fountain, 31 August 2016), split between the keyboardist's compositions and his own plus covers of Herbie Hancock, Pat Metheny and Ivan Lins. Izumi passed away in April 2021, and Sutoh is credited as bassist and recording engineer on two tracks of the posthumous Unforgotten Saga (Misty Fountain, 2022).

Since 2022 he has worked in two new groups. Akasaka Trio, formed in 2022 with Andoh and Noritake, is one. The other, 4GENEXYZ, grew out of a July 2022 session with Noritake, the trumpeter Sora Fujii, the saxophonist Takafumi Kawashima and the keyboardist Jun Tomoda, and takes its name from the four generations in the band, born in 1994, 1984, 1977 and 1964. It released Sky, River and Friends on the Earth on 2 October 2024 and a second album, Neostalgia, on 7 October 2026. He is 61 and still touring heavily. His own calendar lists a date with Honda's band at the Jazz-Fusion Summit in Tokyo on 1 November and a ten-day tour from 5 to 14 December marking forty years since he joined The Square, ending in Tokyo on his 62nd birthday.

Sutoh is a fusion bassist, and Discogs files nearly all of his own and band records under Jazz with the Fusion style, a few of them also tagged Smooth Jazz. Inside T-Square he was the low end of a pop-minded fusion band, the bass under Andoh's guitar and the horn and keyboard themes, and the credit "Bass [Solo]" on T Comes Back (2003) shows the group giving him room to step out. He is also a working composer: the pieces he wrote for the band, among them "Shadow," "Nab That Chap!!," "Sunnyside Cruise" and "Our Fortress," and for Trix over two decades, sit in the same song-like format as the rest of the repertoire.

Alongside the fusion work he is a first-call session player for Japanese pop and vocal acts, including long touring with The Gospellers, and he has recorded for rock, game-music and anime-theme projects. That work is real, but the great majority of his releases are fusion, and Discogs and Japanese Wikipedia both place him there first.

His sound is documented in some detail. In 2022 he used mainly Moon JB-5 five-string basses, one active and one passive, a fretless JB-5, and a Moon MBC-6 six-string. He sets the action high so the instrument rings, aims for an uncompressed direct amp tone, and uses a Boss GT-1000 and an Akai UB-1 when he wants modulation or effects, plus a Roland V-Bass with the six-string.