PURPOSE: To make forming and assembling easier by a method in which a rotary part to be turned around a rotary shaft and a fixed shaft in relation to a fixed part is actuated toward opening direction by a torsion bar to be provided with initial torque by disengagement of the fixed shaft.

CONSTITUTION: A through hole 13 is formed in one of rotary parts 10 and a fixed part 30, and a rotary shaft 40 is pivotally supported on the others and rotatably attached to one end of the hole 13. A fixed shaft 50 is detachably coupled to the other part in a tunable manner and rotatably inserted into the other end of the hole 13. A torsion bar 20 is set between the hole 13 and the shaft 50 to provide it with initial torque by rotation with the disconnection of the shaft 50. The parts 10 can thus be turned around the rotary shaft 40 and the fixed shaft 50 in relation to the part 30 and can also be actuated to the opening direction by the bar 20.