The present invention pertains to a muscle synthesis promoter which exhibits a muscle synthesis promotion action, and with which it is possible to efficiently ingest milk protein. More specifically, the present invention pertains to a muscle synthesis promoter containing, as active ingredients, 0.2-1.2 wt.% of a soybean polysaccharide, 0.1-0.5 wt.% of a HM pectin, 0.01-0.1 wt.% of a fibrous insoluble cellulose, and an acidic liquid composition that contains a milk protein mixture including a casein protein and a whey protein. This muscle synthesis promoter is capable of improving the muscle synthesis promotion effect by preventing the curdling of the milk protein and promoting the absorption of the milk protein in the stomach.
