PURPOSE: To provide the mold clamping device of an injection molding machine capable of improving the accuracy of stoppage position of a movable platen accompanied by a mold opening and closing operation.

CONSTITUTION: The device includes a control valve 27 for supplying oil to the oil chamber of a mold opening and closing cylinder 16, wherein the movable platen can be extended and retracted by feeding and discharging oil to the oil chamber. A position detector 31 detects the position of the movable platen and outputs position signals Srb, and a position pattern generator 35 outputs position pattern output Sref. Then the position signal Sfb is fed back to the position pattern output Sref so that a command value (m) is generated to be output to the control valve 27. Furthermore, the device has an automatic NULL controlling valve 57, and at the time of the achievement of set boundary conditions, a NULL control is executed by adding an offset quantity to the command value (m).