The problem addressed by the present invention is to provide: a method for safely and effectively intensifying the physiological action of caffeine at low levels of intake when utilizing the physiological actions of caffeine resulting from exciting the central nervous system and increasing mental function； and a composition for improving physiological function having caffeine of which the physiological action has been intensified as the active component. As a solution, the physiological action of caffeine is intensified by combining caffeine with ornithine or a salt thereof. Also, provided is a composition for improving physiological function having caffeine as the active component by means of combining caffeine and, as an agent for intensifying the physiological action of caffeine, 0.1-100 parts by weight of ornithine or a salt thereof as the quantity of free ornithine for every 1 part by weight of caffeine. An improvement in physiological function by means of an action of reducing mental fatigue, an action of preventing drowsiness, or an action of sustaining concentration can be cited as the improvement in physiological function resulting from caffeine in the present invention.
