A polymethylsilsesquioxane powder surface-treated with an organosilicone compound represented by the formula:(R.sup.1.sub.a R.sup.2.sub.3-a Si).sub.b Zwherein R.sup.1 represents an alkyl group substituted by a perfluoroalkyl group; R.sup.2 represents an unsubstituted monovalent hydrocarbon group; a is an integer of from 1 to 3; b is 1 or 2; and when b is 1, Z is selected from the group consisting of a hydrogen atom, a halogen atom, a hydroxyl group, --OR.sup.3, --NR.sup.3 X, --ON(R.sup.3).sub.2 and --OCOR.sup.3, and when b is 2, Z is selected from the group consisting of --O--, --N(X)-- and --S--, in which R.sup.3 represents an alkyl group having 1 to 4 carbon atoms and X is selected from the group consisting of a hydrogen atom and an alkyl group having 1 to 4 carbon atoms.