To provide a structure of fitting a wall panel to a strut in a simplified structure, for simply fitting the wall panel to the strut, simplifying the manufacturing of the strut, providing the strut at a low price, attaining a favorable appearance and preventing the lowering of strength of the strut.

The strut 10 is fixed to a lower frame 30 with a bolt 12 in a fitting piece 11. An enclosure 60 is mounted on the interior side of the strut 10. The enclosure 60 is composed of: a strut fitting part 61 formed wider than the strut 10, a wall panel fitting part 62 formed to be bent on both sides of the strut fitting part 61; and a cover fitting part 63 formed to be bent at the end of the wall panel fitting part 62. The strut fitting part 61 is fixed to the strut 10 with a screw 65. A wall panel 20 is fixed to the wall panel fitting part 62 with a stud bolt 66 and a nut 67. A cover 70 is fixed to the cover fitting part 63 with a screw 68. A screw cover 72 is fitted to the cover 70.