An object of the present invention is to provide a prophylactic or ameliorating agent for a genetic disease that is caused by a mutation in an exon of a gene and is capable of forming a functional truncated protein by skipping of the exon comprising the mutation. The prophylactic or ameliorating agent used in the present invention is a prophylactic or ameliorating agent for a genetic disease that is caused by a mutation in an exon of a gene and is capable of forming a functional truncated protein by skipping of the exon comprising the mutation, wherein the prophylactic or ameliorating agent contains a compound having a molecular weight of 1500 or lower.