A main object of the present technology is to provide a composition for muscle mass increase capable of effectively increasing muscle mass with fewer side effects and high safety. A composition is provided for increasing muscle mass containing Bifidobacterium and/or a culture thereof as an active ingredient； a method is provided for increasing muscle mass in which Bifidobacterium and/or a culture thereof is administered as an active ingredient； a method is provided for improving or treating muscle diseases or symptoms thereof, in which Bifidobacterium and/or a culture thereof is administered as an active ingredient； Bifidobacterium and/or a culture thereof is provided for improving or treating muscle diseases or symptoms thereof； use of Bifidobacterium and/or a culture thereof is provided for increasing muscle mass； and use of Bifidobacterium and/or a culture thereof is provided for producing a composition for increasing muscle mass containing the Bifidobacterium and/or a culture of thereof as an active ingredient.
