A stationary die is held on the front surface of a stationary platen and a movable die is held on the front surface of a movable platen. A plurality of monitoring sections are set in advance within a moving range of the movable platen and an upper limit of forward thrust of the movable platen is set for each of the monitoring sections. When the forward thrust of the movable platen reaches the upper limit set in one of the monitoring sections during a die closing operation, it is determined that abnormality occurs to stop the movable platen from moving forward. If a die closing mechanism is of a toggle link type, the forward thrust of the movable platen is calculated based on the thrust of a crosshead which drives a toggle link mechanism, the moving speed of the crosshead, and the moving speed of the movable platen.