The hydraulic system comprises a 4/4-way valve, which is adapted to control the velocity of the mold-closing operation and the fluid pressure during said mold-closing movement and to assume three consecutive control positions, in each of which fluid pressure is applied to a stationary piston of an advance-retract cylinder from a first cylinder chamber, which is nearer to the mold. One of said control positions is a differential control position, in which pressure fluid displaced from the second cylinder chamber of said advance-retract cylinder, which second chamber is remote from the mold, is supplied to a pressure conduit through a branch passage, which includes a non-return valve. The differential control position is preceded and succeeded by a transitional control position, in which the pressure fluid displaced from the second cylinder chamber is returned through the 4/4-way valve to a reservoir. As a result of that arrangement the production rate of the injection molding machine can considerably be increased whereas only gradual velocity and pressure chambers are effected during the mold-closing movement.