To dispense with a pad material to reduce the weight, to prevent the increase in surface pressure of the side parts of the seating part of a facing material, and to eliminate the cramped or bottomed feeling of both side parts of the hip or back.

A seat frame 21 is formed of a pipe frame. The facing material 25 is formed of a three-dimensionally structured textile fabric consisting of a surface foundation cloth 26 and back foundation cloth 27 of surface and back textile fabrics and an elastic intermediate layer 28. Cutout parts 33 are formed on both side parts of the seating part of the facing material 25. Over- lock parts 37 as decorative parts are formed on the peripheries of the cutout part 33. A facing material 39 more stretchable than the facing material 25 is mounted on the cutout parts 33 of the seating part of the facing material 25. The seat frame 21 exposed from the cutout parts 33 of the seating part of the facing material 25 is covered with a protective material 35.