PURPOSE: To obtain repeatedly stable and fixed mold clamping force.

CONSTITUTION: In a performing method of mold clamping control of an injection molding machine by making use of a mold clamping ram 7 to be connected with a moving die plate 2, a mold clamping cylinder 6 in which the mold clamping ram 7 is fitted, a boost cylinder device moving the moving die plate 2 and a prefilling valve 10 to be provided between the mold clamping cylinder 6 and an oil tank 12, the moving die plate 2 is advanced to a fixed advancing limit position by operating the boost cylinder device. Then after detection that the prefilling valve 10 is closed by a detecting device 35. hydraulic oil is supplied to the mold clamping cylinder 6 and the mold clamping is performed by allowing pressure to rise up to preset pressure.