PURPOSE: To simplify the adjusting work by constituting the device so that an inclination angle at the time of erection of a turning supporting lever and a seat attached thereto can be adjusted simply by only adjusting a position in the forward and the backward directions of a pivot of a lock lever by a pivotal position adjusting means.

CONSTITUTION: An engaging groove 36 which is engaged to a detaining shaft 20 and detains a tuning supporting lever 8 to an erect position, and a detaining piece 37 which abuts on the detaining shaft 20 and obstructs the turning supporting lever 8 from turning backward are provided integrally on a lock lever 33. In such a state, an inclination angle at the time of erection of the turning supporting lever 8 and a seat 14 attached thereto can be adjusted by adjusting a position in the forward and the backward directions of a pivot 34 of the lock lever 33 by a pivotal position adjusting means 43. That is, it is unnecessary to adjust separately a fitting position of the lock lever 33 and a position, etc., of a stopper for stopping the tuning supporting lever 8, therefore, the adjusting work can be simplified remarkably.