The present invention relates to a surface-treatment agent for powders for use in cosmetics, including a specific polymer having a specified carbosiloxane dendrimer structure, as well as a cosmetic comprising a powder which has been treated with the above surface-treatment agent. The surface-treated powder does not generate hydrogen, and therefore is safe. Further the surface-treated powder can exhibits, good compatibility with other cosmetic raw materials, and thereby, the stability of the formulation in cosmetics can be improved. The surface-treated powder can provide cosmetics with good water resistance, good sebum resistance, good glossiness, a good tactile sensation, good adhesive properties to hair and/or skin, or the like. The cosmetic can also have superior surface protective properties, a superior outer appearance, and/or a superior sensation during use.
