An object is to provide an external preparation for skin preferably usable to prevent or ameliorate pigmentation. The object is achieved by providing a preventing or ameliorating agent for pigmentation, consisting of a compound represented by the following general formula (1), an isomer thereof, and/or a pharmacologically acceptable salt thereof, and an external preparation for skin containing the same as a component:[wherein R1 represents an unsubstituted aromatic group or an aromatic group having any substituent R2 represents a hydrogen atom, a linear chain or branched chain alkyl group having a number of carbon atom or atoms of 1 to 4, or an acyl group having a linear or branched alkyl chain having a number of carbon atom or atoms of 1 to 4 and R3 represents a hydrogen atom or a linear chain or branched chain alkyl group having a number of carbon atom or atoms of 1 to 4.]
