There are provided a first process T1 in which a mold clamping motor 4 is driven-controlled so as to move a cross head 5h of a toggle link mechanism 5 to a preliminary position Xr set in advance so as to become a position on the mold open side rather than a mold closure position Xs; a second process T2 in which after the first process T1 is finished, a mold thickness adjusting motor 2 is driven-controlled so as to move a pressure receiving platen 3 forward to a mold closed position Xc where a mold C is closed; a third process T3 in which after the second process T2 is finished, the mold clamping motor 4 is driven-controlled so as to move the cross head 5h forward, while the mold C is pressurized by torque limitation of the mold clamping motor 4, and the mold thickness adjusting motor 2 is driven-controlled so as to move the pressure receiving platen 3, while the cross head 5h is moved to the mold closure position Xs; and a fourth process T4 in which, after the third process T3 is finished, a clamping margin Lp of the mold C corresponding to a predetermined mold clamping force is set.