Disclosed is a bacterium belonging to the genus , which can enhance the function of a Peyer's patch to stimulate the production of IgA and can colonize in a human intestinal tract. Also disclosed is an IgA production promoter comprising a cell of the bacterium belonging to the genus as an active ingredient. Specifically disclosed is an IgA production promoter comprising a cell of , particularly a cell of strain CP1750 (FERM BP-10532), as an active ingredient.