To fit an opening/closing door to a box body with a simple action by making a shaft member provided on an upper hinge member movable between the protruded position and the storage position, and invariably exciting it to the protruded position.

The support member 22 of an upper hinge member 11 is fixed at the edge section of an opening/closing door 3, and a shaft member 12 protruded on the support member 22 is rotatably coupled and fitted to the bearing hole 35 of an upper bearing member 13 fitted on the lower face side of the upper plate of a cabinet 1. The shaft member 12 is fitted to the support member 22 movably in the axial direction between the protruded position couplable with the bearing hole 35 and the storage position releasable from the bearing hole 35. The shaft member 12 is invariably excited to the protruded position side via an exciting means 31 constituted of a compression coil spring. When the opening/closing door 3 is merely moved while a lower shaft member 42 is coupled with a lower bearing member 44, the upper shaft member 12 is coupled with the upper bearing member 13, and the opening/closing door 3 can be easily assembled.