To provide a pressure release control method for the mold clamping hydraulic pressure for carrying out the pressure release control for a mold clamping device stably and quickly.

A method comprises steps in which the hydraulic pressure is applied continuously and constantly to a booster ram 9 before the completion of the cooling process for molten resin filled in a cavity of a mold 2, and the hydraulic pressure in a main cylinder chamber 14 is returned to the tank side by quick pressure release circuits 22 and 26, and also the hydraulic pressure in the booster ram 9 is returned to the tank side during the mold opening delayed operation before the opening of the mold and a prefill valve 28 communicated with the main cylinder chamber 14 is opened. The mold clamping force of as main ram 7 is set in the range of 5%-12% to the mold clamping pressure at the time of the cooling process for the mold, and the quick pressure release time for the main ram 7 is set within 0.05 seconds.