To provide an unlocking operation for a cover plate necessary for extracting a sloped plate.

A stopper lever 7 horizontally rotated interlockingly with the opening and closing of the cover plate 1 installed at the front of a box is installed at the inside front part of the box, and a stopper 12 is projectedly installed on the upper surface of the stop lever 7. Thus, when the cover plate 1 is closed after the sloped plate 3 is stored, the stop lever 7 is forward rotated (closing rotation) to position the stopper 12 at the tip side of the sloped plate 3. When the cover plate 1 is opened, the stop lever 7 is reversely rotated interlockingly to disengage the stopper 12 to allow forward moving of the sloped plate 3. As a result, the cover plate 1 is automatically unlocked by merely opening and closing the cover plate 1.