PURPOSE: To utilize the gravity acting on a back rest to erect a seat and facilitate the removal and attachment of the seat by eliminating a connection of the back rest and seat and causing a first slide pin provided on the back rest and a second slide pin provided on the seat to engage a slot for pressing against each other.

CONSTITUTION: A force of a back rest 4 to descend by the gravity is transmitted from a first slide pin 62 engaging a slot 61 to a second slide pin 63 so as to depress the slide pin 63. When a force to hold a seat 3 at the sitting position is thus released, the slide pin 63 is depressed downward along the slot 61 by the slide pin 62 and the seat 3 is erected from the use position to the rising position by the rising action. Further, the support shaft 32 of the seat 3 is slidably back and forth journalled by a support 2 and urged rearward elastically by a spring 5, so that the slide pin 63 in the vertically extending slot 61 is urged upward or downward selectively at a border of a consideration point (a), and thus the seat 3 is not immediately operated even if one leaves the seat. Then the seat 3 rises upward by the own weight of the back rest 4 on the stage of slightly lifted seat 3.