An outer frame has an outer frame upper side plate, an outer frame lower side plate, an outer frame left side plate, and an outer frame right side plate. In an upper right corner, a transparent upper side plate, a transparent right side plate, and holders are applied to inner surfaces of the outer frame upper side and the outer frame right side plate, and are held together by a corner joint and a corner bracket that are inserted into a right end of the outer frame upper side plate and the outer frame right side plate. Upper left, lower left, and lower right corners are similarly assembled.