PROBLEM TO BE SOLVED: To provide a novel muscle mass-increasing agent derived from edible natural products.SOLUTION: The muscle mass-increasing agent of the invention contains as active ingredients black ginger (Kaempferia parviflora) and/or black ginger (Kaempferia parviflora) extract, or methoxy flavone represented by chemical formula 1 (where, R, R, and Rare all -H or -OMe, and Ris -OH), particularly, one or more compounds selected from the group consisting of 5-hydroxy-3,7,3,4-tetramethoxyflavone, 5-hydroxy-7-methoxyflavone, 5-hydroxy-3,7-dimethoxyflavone, 5-hydroxy-3,7,4-trimethoxyflavone.