Abstract: The present invention provides a pharmaceutical composition effective for treatment of the PolyQ diseases, which can be safely administered to humans. The present invention provides a pharmaceutical composition comprising: one selected from the group consisting of arginine, a physiologically acceptable salt thereof, and a solvate thereof, as an active ingredient, in which the pharmaceutical composition is used in treatment or prevention of the PolyQ diseases, and the the PolyQ disease is one selected from the group consisting of Huntington disease, inherited spinocerebellar ataxias, dentatorubral palliodoluysian atrophy, and spinal bulbar muscular atrophy.