PURPOSE: To perform the disjointing of a mold and the cutting of a gate in series and to prevent plastic from remaining in a sprue bush by starting the sliding of a gate block by a slide core after upper and lower molds are separated.

CONSTITUTION: A slide core 8 is engaged with a gate block 10 in such a state that a play 11 having a predetermined interval is provided along a moving direction so as to start the sliding of the gate block 10 after an upper mold 20 and a lower mold 2D are separated. For example, a rod 12 having a T-shape head 12a is integrally attached to one end of the gate block 10 and, when the slide core 8 is moved in such a state that the head 12a is in contact with the engaging hole 13 formed to the slide core 8, the gate block 10 is pulled. There is the play 11 between the engaging hole 13 and the head 12a and the gate block 10 is not slid until the slide core 8 is moved over the distance corresponding to the play 11.