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 heterocyclic compound represented by formula (1) or a pharmacologically acceptable salt thereof: wherein R1, R2, and R2′ are each independently C1-3 alkyl; Y is S or O; and p is an integer of 0 to 3, wherein when p is 2 or 3, R1 may be the same or different.