an automatic mold thickness adjusting method is provided , which is capable of effecting mold thickness adjustment in a short time by a low - cost mold clamping apparatus using a general - purpose motor for mold thickness adjustment , and of preventing an overcurrent from flowing in the motor . a crosshead is retreated to a position behind a position for production of a set mold clamping force when the crosshead reaches a lock - up position before a die - touch state is attained , or when the die - touch state is established in a crosshead position before the aforesaid position , and a rear platen is advanced so that the die - touch state is attained . thereafter , the motor for mold thickness adjustment is driven in a direction to retreat the rear platen , and at the same time , a servomotor is driven in a direction to advance a movable platen , at a speed equal to the retreating speed of the rear platen obtained by multiplying a program - specified speed by an override value corresponding to a crosshead shift position , thereby causing the crosshead to reach the aforesaid position .