By forging a metal material, there is formed a dustproof boot 20 in which a side of an opening end portion 20 b formed with a groove 34 for a dustproof boot is provided with a diameter smaller than that of a side of a piston main body 20 a, and a total thereof is provided with a substantially uniform wall thickness, thereafter, by carrying out ironing from a front end of the opening end portion 20 b to the piston main body side, the recess groove 34 for the dustproof boot can be formed by providing a build-up portion 20 c at the opening end portion.