PURPOSE: To prevent the generation of 'short' at the time of initial weighing operation while suppressing the wasteful outflow of a resin from an injection nozzle.

CONSTITUTION: At the start time of operation, a screw 12 is allowed to retreat to the weighing set position S9-α set immediately before a weighing completion position S9 to perform suck-back and rotated in a back pressure free state to perform weighing. Next, the screw 12 is allowed to retreat to the weighing completion position S9 and subsequently allowed to retreat to the suck-back position S10 set behind the weighing completion position S9 to perform suck- back. In this case, the screw 12 is allowed to finally arrive at the weighing completion position S9 by the pressure of the resin charged in a heating cylinder 11 and, therefore, the heating cylinder 11 is sufficiently filled with a resin at the time of the completion of weighing. It is prevented that mold release deficiency caused by the generation of 'short' is brought about.