A back surface of an outer case is formed by mutually overlapping left and right back panels. The overlapping edges on the front side and backside of the panels may be bent toward the opposite side at a blunt angle. A joint between the back panels is established in which the overlapping edge of one back panel is pressed against the overlapping edge of the other back panel. Spacers are mounted between the outer and inner case such that the overlapping edges are put in close contact with each other. The bends are deformed in order to prevent leakage from the joint after the injection of foam liquid. Due to the pressure of the foam, both of the overlapping edges are placed in closer contact while deforming the bends to be substantially flat. Consequently, the sealing properties of the joint after the completion of the thermally insulated housing are secured.