To miniaturize a vehicle seat folding mechanism by reducing the number of components thereof as much as possible.

An engagement projecting part 20 which is integrally rotated with an arm plate 14 is provided on the arm plate 14. Pulling members (30, 40) which are turned by pressing the engagement projecting part 20 to pull an unlocking cable 10, and a stopper member 50 having an abutting surface 52 to be abutted on the pulling members (30, 40) are provided on one side of a base plate 12 facing the engagement projecting part 20. By tilting a seatback 6 forwardly, the pulling members (30, 40) are turned by pressing the engagement projecting part 20, the pulling members (30, 40) are abutted on the abutting surface 52 of the stopper member 50 while pulling the unlocking cable 10, and the seatback 6 is kept in a forwardly tilted state. By releasing the abutting of the pulling members (30, 40) on the abutting surface 52, the seatback 6 in the forwardly tilted state is displaced in a largely collapsed posture.