Abstract: An object of the invention is to provide a novel technique for preventing muscle atrophy by directly promoting muscle synthesis or suppressing muscle degradation without through physical activity. An agent for preventing muscle atrophy containing a lactic acid bacterium having a muscle synthesis-promoting effect and/or a muscle degradation-suppressing effect, a treated product of the lactic acid bacterium or an extract thereof as an active ingredient in which the lactic acid bacterium having a muscle synthesis-promoting effect is Lactobacillus gasseri or the like and the lactic acid bacterium having a muscle degradation-suppressing effect is Lactobacillus reuteri or the like is provided.