PURPOSE: To obtain an external preparation for inhibiting melanin formation which is used in treatment and prophylaxis of pigmentation such as chloasma and lightening human skin by using N-acyl-O-acylmannosamine or N-acyl-O- acylgalactosamine as an active ingredient.

CONSTITUTION: The subject preparation contains 0.001 to 20wt.%, preferably 0.01 to 10wt.%, based on the total amount of the preparation, of N-acyl-O- acylmannosamine, for example, N-acetyl-1,3,4,6-tetra-O-acetylmannosamine or N-acyl-O-acylgalactosamine, for example, N-acetyl-1,3,4,6-tetra-O- propionylgalactosamine. The acyl groups in the compounds as an active ingredient are preferably those which are derived from fatty acids of 2 to 21 carbon atoms.