To solve the problem of a difficulty in the securing of the machining precision for two layers, inner and outer, of gear trains and the distance between the shaft centers and the decrease in the friction resistance while being effective for the higher strength of the meshed parts of the gear teeth in the adjustment of the angle of inclination with respect to the seat back and the seat cushion with the two layers, inner and outer, of gear trains.

The pitch circles of external tooth gears (13 and 14) are made common between both, inner and outer and the pitch circles of internal tooth gears (11 and 12) made common between both, inner and outer. The backlash of the inner and outer gears (8 and 7) are made larger and normally, for example, the torque is transmitted with the inner gear (8) and when a large extraneous force works, it is received by the outer gear (7) and the two layers of gear trains.