PURPOSE: To accurately close a flapping door by hooking the hooking pawl which is freely oscillatingly supported on a packing box side using the push member with a spring and releasing the engagement using the second spring.

CONSTITUTION: When the packing case 2 is dumped backward, the pushing member 8 is lifted by the force of the first spring 12 together with the packing box. Until the nut 15 and the guide cylinder 9 make contact with each other, the working section 6 and the pushing member 8 make contact. The engagement of the hooking pawl 6 and the hooking member 7 are maintained and subsequently the push member 8 and the working section 6 are separated. When the hooking pawl 6 turns counterclockwise by the second spring 14, its engagement against the hooking member 7 is disengaged and the rear flapping door is released. Then, as the packing box 2 is let to fall, the working section 6 makes contact with the push member 8 and the hooking pawl turn clockwise. Also, the hanging section 6' engages with the hooking member and the rear flapping door 5 closes. Then, the push member 8 is let to fall and the packing box 1 is fixed on the frame.