wired board fixedly mounted to a back of the shelf unit;
a sub back wired board positioned further to the rear of the main back wired board;
a plurality of connectors for plug-in units mounted on an inner surface of the main back wired board so as to be connected to the plug-in units inserted into the shelf unit;
a plurality of connectors for external cables mounted on an outer side of the sub back wired board so as to be connected to external cables;
a plurality of relay connectors mounted on a surface of the main back wired board, the surface opposing the sub back wired board, and electrically connected to the plurality of connectors for plug-in units; and
a plurality of relay connectors mounted on surface of the sub back wiredto be connected to the plug-in units inserted into the shelf unit;
a plurality of connectors for external cables mounted on an outer side of the sub back wired board so as to be connected to external cables;
a plurality of pin terminals provided on the plurality of plug-in unit connectors, tip portions of the pin terminals being inserted into and projected through the main back wired board; and
a plurality of relay connectors mounted on a surface of the sub back wired board, the surface opposing the main back wired board, the plurality of relay connectors being electrically connected to the plurality of connectors for external cables,
wherein tip portions of the plurality of plug-in unit connector pin terminals are connected to the plurality of relay connectors on the sub back