PURPOSE: To eliminate mold mismatch flaws and liquid leakage by discretely forming the collector member and holding member of a plunger from pull molds which are respectively discretely shaped.

CONSTITUTION: The hollow plunger has the collector member 14 corresponding to a first member and the top end thereof is closed by a cap 16. The holding member 18 corresponding to a second member is formed to a crown shape on the other side of the collector member 14. The collector member 14 and holding member 18 of the plunger are discretely formed from the pull molds which are respectively discretely shaped in such a manner that the toric shoulder part 23 of the collector member 14 is cooperative with the toric axial end face of the holding member 18 so as to be capable of forming a toric groove to be arranged with an O-ring. As a result, the mold mismatch flaws and the liquid leakage may be eliminated.