A connecting cylindrical part of a second hinge member is inserted between first and second support arm parts of a first hinge member such that an axis of the second hinge member is aligned with that of a rotation axis. A hinge cylinder is passed through the connecting cylindrical part. The hinge cylinder is non-turnably fitted to opposite end parts of the connecting cylindrical part. The opposite end parts of the hinge cylinder projecting from the connecting cylindrical part are turnably fitted to the first and second support arm parts through bearings, respectively.