Disclosed is an external preparation for skin, which is suitable for the prevention or amelioration of pigmentation. Specifically disclosed are: a prophylactic or ameliorating agent for pigmentation, comprising a compound represented by general formula (1) or an isomer and/or a pharmacologically acceptable salt thereof； and an external preparation for skin, containing the component. [In the formula, R1 represents a unsubstituted aromatic group or an aromatic group having a substituent； R2 represents a hydrogen atom, a linear or branched C1-4 alkyl group, or an acyl group having a linear or branched C1-4 alkyl chain； and R3 represents a hydrogen atom, or a linear or branched C1-4 alkyl group.]
