PURPOSE: To lessen resin burrs on the outer periphery of a plunger head by a method wherein a catching member for separating a seal member and the plunger head from a plunger main body is interposed between the seal member and the plunger main body.

CONSTITUTION: An annular catching member 10 capable of making a plunger head 1 rotate is incorporated between a plunger main body 3 of a resin pouring part and the plunger head 1 of the resin pouring part. The member 10 is provided with notches 10A at the opposed parts of its peripheral surface and is provided with a square hole 10B at its central part. The intermediate part IC of the head 1 is processed in such a way as to fit into this square hole. A spanner 11 is engaged with the notches 10A of the member 10 and is rotated. Thereby, a seal member 2 and the plunger head 1 are separated from the main body 3 and the assembly and the disassembly of the member 2 are executed. According to such a way, as the thickness of the head 1 can be made thin and a recessed part for rotation and catching becomes unnecessary, resin burrs on the outer periphery of the head can be lessened.