To accelerate a tilt linked with a raising/lowering drive of a seat surface by a simple constitution of adding some elements to a general raising/lowering chair, reduce a physical burden of a person in sitting/standing, and add an adjusting function of a seating posture thereto.

A connecting board 60 rotatably attached to a fixing part 21 of a drive mechanism 20 fixed to a chair leg part 10 is provided with a bearing part C61, a raising/lowering movable part 22 fitted to the fixed part 21 is attached with a raising/lowering base part 30 with bearing parts A31 and D32, a seat part 40 with parts to be born A41, B42 and E43 is disposed in its upper part, and an auxiliary base part 80 with parts to be born D81 and F82 is disposed its lower part and extends in the front/rear direction. A connecting rod 50 with turning fulcrums B and C and an auxiliary rod 85 with turning fulcrums E and F are provided with length adjusting devices 51 and 86 respectively and disposed in the front and the back respectively. The respective parts A-F are turnably connected to one another and linked with the raising/lowering of the movable part 22 to tilt the seat part 40.