During a mold thickness adjustment process of an injection molding machine, a mold touch position at which the mold halves of a mold contact each other is detected, and the mold temperature is detected by means of thermocouples attached to the mold halves, respectively. Also, the mold thickness is obtained based on the position where a movable platen and a stationary platen of the injection molding machine contact each other, which is obtained previously, and the detected mold touch position.After a molding operation is started, the mold temperature (Tn1, Tn2) is detected in each of molding cycles, and the amount of change of the mold thickness (.DELTA.e) during the period between the preceding cycle and the current cycle is calculated (S14) based on the change of mold tempreature (.DELTA.T) during the period between the two cycles, the mold thickness (e), and the thermal expansion coefficient (k). The mold touch position for the current cycle (P) is obtained (S16) based on the calculated value. When the mold is clamped in the current cycle, the movable platen is moved from the mold touch position toward the stationary platen by a predetermined amount, to produce a predetermined mold clamping force.