The development of a skin-whitening agent which has a lower molecular weight, can be synthesized through a fewer steps and can be produced at low cost has been demanded. Thus, it is provided by the present invention a skin-whitening agent comprising one or more compounds selected from the group consisting of 4-cis-hydroxyproline and a derivative and/or a salt thereof, a skin-whitening composition for external formulation to the skin, a cosmetic composition and a pharmaceutical composition, each of which contains the skin-whitening agent of the invention, and a cosmetic method for whitening the skin, which comprises a step of administering one or more compounds selected from the group consisting of 4-cis-hydroxyproline and a derivative and/or a salt thereof. In the skin-whitening agent and the cosmetic method for whitening the skin of the invention, the 4-cis-hydroxyproline may be 4-cis-L-hydroxyproline and/or 4-cis-D-hydroxyproline.
