PURPOSE: To simplify the operation and improve the working efficiency, and to prevent a damage and the like of the apparatus, by installing a tortion spring member to energize an upper structure in the opening direction to the shaft of the upper structure, and fixing the lower linear part of the tortion spring to a lower structure.

CONSTITUTION: To the shaft 5 of an upper structure 3, a tortion spring 10 to energize the upper structure 3 in the opening direction is installed rotatable, the lower linear part of the tortion spring 10 is fixed to a lower structure 1, and its upper linear part 17 is inserted to the long hole 20 of a metal fitting of the upper structure 3. And the structure 3 is opened up to the first specific angle (a) by the balancing relation between the spring constant of the spring 10 and the dead load of the structure 3. Furthermore, when the structure 3 is rotated up to the second specific angle (b) where the position of the gravity center G of the structure 3 operates the dead load in the opening direction, an elastic energy against the operation in the opening direction is accumulated by a deformation of the spring 10 by a reserve energizing member. Consequently, shocks can be avoided without providing a stopper.