To prevent the falling of the mold fastened to a mold attaching plate by employing a clamp or a magnet plate by the connection of the key groove and key member provided to the contact surfaces of both of them at the same position.

Key grooves 5, 6 are provided to the contact surfaces of the mold attaching plate 3 of the fixed plate 1 on a machine stand 2 and the mold 4 fastened to the mold attaching plate 3 by a clamp or a magnet from both side ends thereof to the in-plane central parts thereof so that the positions of the key grooves 5, 6 become same. The key members 7 having a cross-sectional shape same to that of the hole parts 8 formed by superposing both key grooves 5, 6 one upon another are inserted in both key grooves 5, 6 from the side end surfaces of the mold attaching plate 3 and the mold attaching plate 3 and the mold 4 are integrally connected to prevent the falling of the mold 4. The pulling-out of the key members 7 is prevented by the clasp jigs 10 provided to the side end surfaces 3a of the mold attaching plate 3.