PURPOSE: To integrally form a lost foam pattern by using a freely attachable and detachable auxiliary mold in order to improve the dimensional accuracy of the lost foam pattern.

CONSTITUTION: The base of a recessed part 2c of a moving mold 2 is provided with a magnet 5, a knock pin 6 and an air vent 7 and the auxiliary mold 3 is provided with a magnetic metal 9 and a knock hole 10. The magnet 5 attracts the magnetic metal 9 and the auxiliary mold 3 engages with the recessed part 2 when the knock pin 6 is inserted into the knock hole 10, by which the auxiliary mold 3 is mounted in the moving mold 2. Pressurizing air is blown from the air vent 7 to the auxiliary mold 3, by which the auxiliary mold 3 is disengaged from the moving mold 2.