Soutaiseiriron is a Japanese band and experimental music project formed in September 2006. Their debut self-released EP Shifon Shugi came out in June 2007. In their early days, all members contributed to songwriting and composition, typically building tracks from shared fragments through improvisational sessions. The lineup and number of members have varied over time, particularly since 2010. Frontwoman Yakushimaru Etsuko, whose father is a scientist, named the band Soutaiseiriron ("Theory of Relativity") and has referred to the group as “software,” reflecting its modular, evolving nature. They operate entirely independently under their own label mirai records, with no affiliation to major labels or production companies. The band is known for maintaining anonymity in visual media—rarely appearing in music videos or on television, and choosing not to engage in typical promotional activities. Members are involved in a wide range of artistic and musical endeavors, including international collaborations. Their performances combine pop structures with experimental elements like Yakushimaru’s original instruments such as the luminous dimtakt and the gesture-controlled YXMR Ghost 'Objet’. These tools, along with improvisation and feedback, create a unique, spacey sound. They often perform at unconventional venues such as art museums. Their single “Synchroniciteen” (2010) remains one of their most well-known releases. Members are Etsuko Yakushimaru (vocals), Seiichi Nagai (guitar), Tadashi Yoshida (bass), and Motoki Yamaguchi (drums).
