to the seat-cushion frame 31 by means of bolts 84, 84 that function as a pair of fixing parts, the base member 8 is also fixed, along with the lower arm 32, to the seat-cushion frame 31. accordingly, there is no need to separately fix the lower arm 32 to the seat-cushion frame 31, and fix the base member 8 to the seat-cushion frame 31, but the lower arm 32 and the base member 8 are fixed together to the seat-cushion frame 31, by means of the bolts 84, 84. in other words, the bolts 84, 84 serve to fix the base member 8 to the lower arm 32, and 