A closing machine for closing a rotating work piece by pressing a heated die against the work piece comprises an air purge box which opens onto the vicinity of the work piece, air supply means for supplying air to the air purge box, and a sensor provided in the interior of the air purge box to detect a state of the work piece or the die. The apparatus includes a substantially rectangular frame member in which a central opening is defined. A predicted increase in driving torque of the ac generator is calculated from a difference between the present driving torque and a driving torque that corresponds to the predicted output current.