portion of the outer cover 11 here the first sliding holes 111 of the outer cover 11 is oriented frontward with reference to the front panel 1 and the second sliding holes 121 of the inner cover 12 are oriented sideward with reference to the front panel 1 as shown in fig1 the first fixing portions 22 are inserted in the first sliding holes 11 and the second fixing portions 21 are inserted in the second sliding holes 121 that is as the first fixing portions 22 are inserted in the upper hole 111 a of the first sliding holes 111 the second fixing portions 21 are simultaneously inserted in the upper holes 121 a of the second sliding holes 121 next as shown in fig6 the side decoration panel 2 slides downward so that the ribs 21 b and 22 b of the first and second fixing portions 22 and 21 can be interlocked with the lower holes 121 b of the first and second sliding holes 111 and 121 thereby completing the assembling process of the side decoration panel 2 with the front panel 1 during the above process since the sliding holes 111 and the sliding holes 121 are alternately formed on the outer and inner covers 11 and 12 respectively a distance between the adjacent sliding holes 111 and 121 is reduced so that the fixing portions 21 and 22 of the side decoration panel 2 are tightly fixed on the front panel 1 according to the present invention since the shaped sliding holes are alternately formed on the front panels and the side decoration panel is provided with the fixing portions.