To enable reduction of a force exerted on a ratchet gear tooth without increasing the size of a mechanism.

The ratchet gear tooth 4 is formed in a gear cutting plate 1 situated entirely individually from a connection member 10 on the seat side and a connection member 20 on the back rest side, and the gear cutting plate 1 is rotatably mounted on a pin 5 situated on the inner surface of the connection member 20 on the back rest side. A working point A of a force F0 exerted on the connection member 20 on the back rest side forms the position of a rotary shaft 5 of the gear cutting plate 1 and a force F2 exerted on the ratchet tooth 4 can be decreased to a low value by setting a distance L1 from a rotary shaft 30 of the connection members 10 and 20 on the seat side and the back rest side in its position to a long length according to the principle of a lever. This constitution enables relative improvement of the withstand load of the ratchet tooth without increasing the size of the whole of a closing mechanism and reduction of a pitch of angle regulation.