PURPOSE: To reduce a space required for a seat rotation by a method wherein a back leaning plate and a seat pad are interlinked with a link and both the back lean ing plate and the seat pad can be retained in a state wherein both are erected and folded.

CONSTITUTION: The lower part of a back leaning plate 3 and the rear upper part of a rotation seat 2 are pivotally attached at a back pivotally attaching point 4 and then, the rear part of a sear pad 5 is pivotally attached to the rotation seat 2 in front of the back pivotally attaching point 4 by means of a seat pivotally attaching point 6. And at the same time, the seat pad 5 front part is supported by a strut 29. Next thereto, an upper pivotally attaching point 7 installed at the back leaning plate 3 above the back pivotally attaching point 7 and a lower pivotally attaching point 8 installed at the seat pad 5 under the seat pivotally attaching point 6 are interlinked through a link 9. Further, a front stopper piece 13 and a rear stopper piece 17 both of which are restricted from release are installed at a fixing base 1 by means of a lever 22 and a concabe part 20 is installed at the lever 22 in order that the lever 22 can interlink with the rear lower end 21 of the seat pad 5 and retains a folding state. When the seat pad 5 is rotated upward around the pivotally attaching point 6, the pivotally attaching point 8 pulls the pivotally attaching point 7 through the link 9 and the back leaning plate 3 rotates around the pivotally attaching point 4, thus, making the seat pad 5 and the back leaning plate 3 erect keeping in a close contact state.