Provided i s a compound having a good skin lightening effect and excellent safety and stability, and an external agent for skin t o which this compound i s added. The compound i s a 4-alkylresorcinol derivative represented b y general formula (I) or a salt thereof. (In the formula, R i i s a branched or cyclic alkyl group or a linear C 2 alkyl group, R 2 and are each independently a hydrogen atom or group represented b y -P(0)(OR )(OR 5) and at least one o f R 2 and i s a group represented b y -P(0)(OR4)(ORs), and and R 5 are each independently a hydrogen atom or linear or branched C 2 alkyl group.)
