The present invention aims to provide a prophylactic or therapeutic agent for idiopathic inflammatory myopathy or idiopathic inflammatory myopathy associated with steroid-induced myopathy that develops during the course of treatment. The present invention provides a prophylactic or therapeutic agent for idiopathic inflammatory myopathy or idiopathic inflammatory myopathy associated with steroid-induced myopathy that develops during the course of treatment, which contains isoleucine, leucine and valine as active ingredients.