To reduce sliding resistance to a seal ring of a caliper and enable good brake performance to be assured by transcribing the surface state of a molding die face applied with crimp processing during molding and forming minute protrusion and recess parts on the surface of the outer circumference thereof in a resin-made disk brake piston.

In sliding of a brake piston and a caliper on a seal ring (rubber) in a brake caliper assembly, a molding die applied with crimp processing during molding is transcribed to reduce their slide resistance and secure good brake performance, and the surface of the outer circumference face thereof is formed as a recess and a protrusion. The surface roughness of this outer circumference face is preferably set in the range of 1.0 to 5.0 S. According to this method, cost reduction can be ensured because polishing can be eliminated, a desired surface roughness can be obtained, and good-quality brake performance (adequate lever stroke and required fluid quantity) can be obtained because the dispersion in the surface roughness is almost eliminated.