Detection method of muscular degenerative diseases, and therapeutic effect judging method

11,15-Dioxo-9α-hydroxy- 2,3,4,5-tetranorprostan-1,20-dioic acid in a specimen isolated from a subject (hereinafter, Tetranor-PGDM) by measuring, muscular degenerative disease the early be detected, as well as to determine the therapeutic effect of the therapeutic agent and / or therapeutic method for these diseases.