PROBLEM TO BE SOLVED: To provide a new active ingredient excellent in the effect of inhibiting phosphodiesterase-3, and thereby a phosphodiesterase-3 inhibitor containing the same.SOLUTION: An isoflavone contained in an arrowroot flower-treated material or in an arrowroot flower, is used as an active ingredient of a phosphodiesterase-3 inhibitor. The arrowroot flower-treated material is preferably consisting of an isoflavone-containing composition obtained from the arrowroot flower. The phosphodiesterase-3 inhibitor is suitably used for the inhibition of the phosphodiesterase-3B.