To provide an injection molding apparatus capable of controlling constantly injection molding pressure.

In a device for feeding a resin for injection molding provided with a cylinder 26 having an injection opening 14 on one end and an insertion opening for freely forward and backward inserting an injection screw 1 on the other end, a hopper 9 for feeding a resin material 11 to the insertion opening side in the cylinder 26, a plunger 40 for injection having a heating cylinder 2 for heating and melting the resin material 11 fed into the cylinder 26, and a rotating and driving part 50 for rotating and driving a spline shaft 4 whose one end is connected with the back end part of the injection screw 1 and other end is connected with the driving shaft 44 of an actuator 15, a pneumatic spring 30 is arranged between the other end of the spline shaft 4 and the driving shaft 44 of the actuator 15.