The present invention has objects to provide an agent for enhancing the effect of skin-whitening ingredients which enhances the skin-whitening action of skin-whitening ingredients and has an improved safeness, and to provide a skin-whitening agent, which contains the above agent and a skin-whitening ingredient(s) and has an improved and enhanced skin-whitening action. The present invention solves the above objects by providing an agent for enhancing the effect of skin-whitening ingredients, which contains one or more members selected from the group consisting of guanine and derivatives thereof as an effective ingredient(s); and a skin-whitening agent which contains the above agent along with a skin-whitening ingredient(s) particularly, members derivatives thereof and/or equol including derivatives thereof.