121 a and 121 b the second sliding holes 121 and the first sliding holes 111 are alternately formed along a vertical length the first sliding holes 111 are formed on the flanges 112 of the outer cover 11 and oriented frontward and the second sliding holes 121 are formed on the bent surface 120 and oriented sideward that is the sidling directions of the sliding holes 111 and 121 are perpendicular to each other meanwhile the side decoration panel 2 has a height same as that of the front panel 1 first and second fixing portions 21 and 22 corresponding to the first and second sliding holes 111 and 121 are formed on an inner surface of the side decoration panel 2 in the vertical direction here the first and second fixing portions 21 and 22 have coupling parts 21 a and 22 a and ribs 21 b and 22 b the ribs 21 b and 22 b are bent from the coupling parts 21 a and 22 a at right angles so that the first and second fixing portions 21 and 22 are not removed from the sliding holes 111 and 121 as the ribs 21 b and 22 b are hooked on the lower holes 111 b and 121 b the bending directions of the ribs 21 b and 22 b of the first and second fixing.