PURPOSE: To facilitate the folding of a seat, by interlocking a swing detector for the bottom of the seat with an unlocking member for the back of the seat so that the seat back can be unlocked only by the swing of the seat bottom.

CONSTITUTION: A lockup member 9 is provided under the bottom 2 of a seat. A hook 20 is provided on a bracket 3 for the pivotal connection of the back 1 of the seat and engaged with a lockup pin 11 to lock up the seat back. The lower end of the hook 20 is coupled to a contact member 30 through a spring 26, a cable 22 and another spring 29. The contact member 30 is nomrally located in contact with a tire housing 7 against the urging force of the spring 29. When the urging force of the spring 29 is made stronger than that of the other spring 26 to disengage the lockup member 9 of the seat bottom 2 from a striker 8 to swing the seat bottom, the contact member 30 is separated from the tire housing 7 to turn the lower end of the hook 20 counterclockwise by the urging force of the spring 29 to unlock the seat back 1.