The purpose of the present invention is to provide a novel therapeutic agent and preventive agent for demyelinating disease. The present invention is a therapeutic agent and a preventive agent for demyelinating disease that contain as an active ingredient a 2-phenylthiazole compound represented by formula (I) or a medically acceptable salt thereof.本發明係以提供新穎的脫髓鞘疾病之治療劑或預防劑為目的。本發明係含有後述式(I)所表示之2-苯基噻唑化合物或此等之醫藥上所容許的鹽作為有效成份之脫髓鞘疾病之治療劑或預防劑。