A problem to be solved by the present invention is to provide a substance having an effect of increasing intracellular ATP and, particular, a potent ATP enhancer far surpassing the increasing effect of inosine alone or a xanthine oxidase/xanthine dehydrogenase inhibitor alone.A human or animal intracellular ATP enhancer comprising a combination of A) and B):A) a xanthine oxidase/xanthine dehydrogenase inhibitor； andB) hypoxanthine, or a compound capable of being converted to hypoxanthine in the body.
