The present invention provides a compound having an excellent inhibitory action on melanin production and being useful as a whitening agent, and a skin external preparation containing the compound. The whitening agent of the present invention comprises, as an active ingredient, a compound represented by formula (1) or a pharmacologically acceptable salt thereof:wherein, R1, R3, R4, and R6 are each independently C1-3 alkyl； and R2 and R5 are each independently a hydrogen atom or C1-3 alkyl.
