It is an object of the present invention to provide: a method for safely and effectively enhancing the physiological action of caffeine even from a low intake of caffeine, when utilized various types of a physiological action of a caffeine which involve stimulation of the central nerve to promote mental function； and a composition for improving physiological function, which comprises, as an active ingredient, the caffeine whose physiological action has been enhanced. As a means to solve the above-mentioned object, the physiological action of caffeine is enhanced by combining caffeine with ornithine or a salt thereof. In addition, the ornithine or a salt thereof in an amount equivalent to 0.1 to 100 parts by weight of free ornithine based on 1 part by weight of the caffeine is mixed with caffeine, as an agent for enhancing the physiological action of caffeine, thereby providing a composition for improving a physiological function comprising caffeine as an active ingredient. The improvement of a physiological function by caffeine in the present invention is the improvement of a physiological function by mental fatigue-reducing action, drowsiness-preventing action, or concentration-maintaining action.
