The present invention relates to a composition for promoting bacterial proliferation for selectively proliferating Lactobacillus casei subsp. casei, which includes a dextran. According to the present invention, a variety of biological activities originated from L. casei subsp. casei can be sustained in a living body by selectively growing-proliferating and colonizing L. casei subsp. casei in the intestine of a human being, animal, or the like or by selectively growing-proliferating L. casei subsp. casei in the intestine, without supplying L. casei subsp. casei at all times.