A composition for promoting bacterial proliferation and selectively proliferating Lactobacillus casei subsp. casei is disclosed, which includes a dextran. A variety of biological activities originating 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.