PURPOSE: To make the calculation and setting of a mold thickness possible with no interposition of worker and consequently contrive to realize efficient mold exchanging operation by a method wherein the mold thickness is automatically calculated and set by utilizing the positional controlling function of a servo motor.

CONSTITUTION: The position of a movable platen 4 at the time when the command current sent from a controlling device 2 to a servo motor 1 exceeds a certain fixed level under the condition that the movable platen 4 and a fixed platen 5 are brought into close contact with each other is stored as the original position or the renewed position (B) of the movable platen in the controlling device 2. Next, positional command, which is sent from the controlling device 2 to the servo motor 1, is changed in succession so as to move the movable platen 4 to the mold opening direction. When the movable platen 4 is moved to the position of a mold opening limit sensor 6 and detected, the controlling device 2 stores the position (A) of the detected movable platen and the movement of the movable platen 4 is stopped. The daylight opening D or the inside dimension between the fixed platen 5 and the movable platen 4 at the mold opening limit position is set in advance in the controlling device 2. Accordingly, the mold thickness M of molds 7a and 7b can be calculated by the equation M=D-(A-B). Thus, the controlling device 2 can automatically calculate the mold thickness M of the molds 7a and 7b on the basis of the above-mentioned equation and set. And the automatic setting of the mold thickness M can be possible.