An injection molding machine is provided with an automatic mold thickness adjusting apparatus. The amount of compensation of the position of a rear platen that is adjusted at the time of installation of a mold is determined by calculating the thermal expansion of the mold that is expected to occur at the completion of a rise in temperature based on the difference in temperature between the mold temperature at the time of mold thickness adjustment and the mold temperature at the time of injection molding. Then, the rear platen is moved to the compensated position thus determined.