[Problem] The purpose of the present invention is to provide a strain of Bifidobacteria having a high serotonin-secretion-promoting ability, and a food and/or beverage containing a treated form of the Bifidobacterium. [Solution] The inventors focused on the fact that serotonin is indispensable for the biosynthesis of BDNF and, as a result of screening numerous Bifidobacteria, discovered a Bifidobacterium derived from the infant intestine having high serotonin production ability specifically, Bifidobacterium longum N61 (NITE BP-02214).