A front panel 11 has a plurality of caulking pieces 16 each of which is formed in a tongue-like shape defined by a pair of slits 17 in a side edge portion 11c of the front panel. The caulking piece is inwardly bent at its base end portion 16a and bent sideward at its distal end portion 16c to form a substantially L-shape. Side panels 12 whose forward end portions are abutted on an inner face 11a of the front panel 11. The side panels are formed along the forward end portions 12a with caulking holes 18 into which the caulking pieces 16 are adapted to be engaged. The front panel 11 is joined to a mount panel 2 in substantially intimate contact along its whole surface. The front panel 11 and the mount panel 2 are formed into an integral structure, whereby the mechanical strength in a direction of the thickness of the front panel can be maintained.