PURPOSE: To enable the title method to provide a bottomed cylindrical piston whose strength is high, by a method wherein a constricted part is formed between a bottom wall of the piston and a sprue congealation to be stuck to the bottom wall.

CONSTITUTION: A bushing 17 is inserted detachably and fixed into a central axis part of a horizontal mold 9 surrounding a right end surface of an inner circumference molding tool 6. A tapered contraction 13 contracting rightward at an angle of about 45 degrees and a tapered sprue 14 expanding rightward from a right end small-diameter part 13a of the contraction 13 are formed continuously, in the central axis part of the bushing 17. A molten plastic material is poured into a cavity 10 through the sprue 14, which is cured and a piston molding 15 is formed. The piston molding 15 is separated from a mold device 5, a cylindrical sprue congealation 15c formed at the sprue 14 is bent and removed from its base part through breaking and a bottomed cylindrical piston 15a is obtained.