相対性理論 (Sotaisei Riron) is a Japanese pop group that formed in Tokyo in 2006 under Mirai Records. Their members are やくしまるえつこ (Etsuko Yakushimaru) (vocals), 真部脩一 (Shuichi Manabe) (bass), 永井聖一 (Seiichi Nagai) (guitar), and 西浦謙助 (Kensuke Nishiura) (drums) who also works as a support drummer for the rock group MASS OF THE FERMENTING DREGS.

The band is known for its clever wordplay in songs and Etsuko Yakushimaru's soft vocals. They are also known for not revealing much about their private lives, refusing to do any interviews of any sort. Taking pictures at their live events are strictly forbidden. To date, 相対性理論 have released four albums: シフォン主義 (Chiffon Shugi, 2008), ハイファイ新書 (Hi-fi Anatomia, 2009), シンクロニシティーン (Synchroniciteen, 2010), and 正しい相対性理論 (Correct Theory of Relativity, 2011).

2006年9月結成
2007年、東京都心にて活動を開始
2008年5月、それまで自主制作していた『シフォン主義』をみらいrecordsよりリリース
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