A problem to be solved by the present invention is to provide a substance having an effect of increasing intracellular ATP and, particularly, a potent ATP enhancer far surpassing the increasing effect of inosine or febuxostat alone.A human or animal intracellular ATP enhancer comprising a combination of A) and B): A) a xanthine oxidase/xanthine dehydrogenase inhibitor or a pharmaceutically acceptable salt thereof; and B) any one or more compounds selected from inosine, inosinic acid, hypoxanthine, and pharmaceutically acceptable salts thereof.