To weaken the existing feeling of a lumber support, and make it less outstanding, and at the same time, prevent unevenness from being generated in the hue of a cushion body, and also, improve the operability of the lumber support and the flexibility or the like of a back plate.

A transparent lumber support 20 having translucency is fitted in such a manner that the vertical location may be adjustable, between the front surface of a back plate 5 and the cushion body 17 which covers the front surface of the back plate 5. In this case, numerous lightening holes 16 are provided on the back plate 5 for a backrest.