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.