The purpose of the present invention is to provide a novel therapeutic or prophylactic agent for tumor lysis syndrome. The present invention is a therapeutic or prophylactic agent for tumor lysis syndrome, which comprises a 2-phenylthiazole compound represented by formula (I) or a pharmaceutically acceptable salt thereof as an active ingredient.本發明係以提供新穎的腫瘤溶解症候群之治療劑或預防劑為目的。本發明係含有以下述式(I)表示之2-苯基噻唑化合物或該等之醫藥上容許之鹽為有效成分的腫瘤溶解症候群之治療劑或預防劑。