PURPOSE: To obtain stable control providing less change in each product-weight by regulating loop gain of a feed back control system for speed, corresponding to injection pressure.

CONSTITUTION: Resistances R3, R4 in addition to feed back resistance R2 in an operational amplifier are equiped. They are constructed such that one end of these resistances are connected commonly, while the other ends thereof are selected by a gain-switch 40. After resin has been filled in a metal mold 25, the gain switch 40 is kept in selecting the resistance 4 until the detected pressure value indicated by a pressure detector 37 reaches first determined pressure value set at low value, and the gain of the operational amplifier is R2/R1. When the detected pressure value exceeds the first determined pressure value, the resistance R3 is selected by the gain switch 40 and the gain of the operational amflifier becomes R2/R1. The gain which is larger than that in the chance when the resistance R2 has been selected, is prepared to a servo valve 35. Thus, when the gain of the operational amplifier is gradually increased, loop gain of speed control is by degrees increased also after the resin has been filled in the mold, thereby compensating the decrease in speed control-accuracy owing to the decrease in current-flow amount of the serbo valve 35.