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 X1, X2, and X3 are each CR2 or N； R1, R2, and R3 are each H, C1-6 alkyl, C1-6 alkoxy, SH, OH, or NRbRc, wherein Rb and Rc are each H, C1-6 alkyl, or hydroxy-C1-6 alkyl, or NRbRc forms a saturated or unsaturated 5- or 6-membered hetero ring； and B is a substituted or unsubstituted pyrazole, imidazole, pyridine, or piperidine； with the proviso that at least one of R1, R3, R4, and R6 is a group other than C1-3 alkyl in the case where X1═X2═Y1═N and X3═CR2.
