28 the top of the plate body 28 is fixed to a transmission shaft 29 the transmission shaft 29 is installed on the upper part on two side walls of the hollow interior 15 so the outside plate 13 is joined by the transmission shaft 29 to the housing 10 the outside cover can swing forwards to extend with the transmission shaft 29 serving as a fulcrum or swing backgrounds to close the upper part of the front opening of the hollow interior 15 between one side of the plate body 28 and the inside wall of the hollow interior 15 is connected with a collapsible connecting rod 30 that will restrict the outside cover 13 to be positioned at an appropriate angle when the outside cover 13 moves forwards to extend the lower part of the plate body 28 of the outside cover 13 forms a lap joint 32 when the outside cover 13 swings backwards to close the upper part of the front opening of the hollow interior 15 the lap joint 32 at the lower part of the plate body 28 of said outside cover 13 can be joined to the lap joint 31 on the upper part of the plate body 24 of the outside ladder at the lower part of the front opening of the closed.