To execute a fastening work for side plates from the inside of a cage to insure safety of an operator and remarkably improve the assembling operation by providing fastening operation holes for fastening the side plates with each other at least on one side of the side plates within mutual side plates to be fastened on the corner parts of a cage.

Within the lateral side plates of a cage, the lateral side plates 4b' fastened with a rear side plate, and the rear side plate 4c' fastened with the lateral side plates are provided with fastening operation holes and recessed parts capable of being fitted on the end parts. The lateral side plate 4b' and the rear side plate 4c' are the side plates to be fastened with each other in the corner part of the cage. For fastening, an operator puts his hands in the fastening operation hole in the cage, and can fasten with bolts. Meanwhile, a finish steel sheet for blockading the fastening operation holes is bent at both end parts and manufactured so as to fit to the recessed parts, inserted to be fitted, and can cover the fastening operation holes.