To adjust the inclination of a seat back by a fine angle and to increase mechanical strength.

An inner tooth string member 120 is fixed to the inside of a seat-side bracket 122. A tooth string 121 of a pitch angle 8° is formed at the member 120. A tooth string member 110 is fixed to the outside of a seat- back-side bracket 112. A tooth string 111 of a pitch angle 6° is formed at the member 110. Then, a lock plate 100 forming the double tooth strings of a tooth string 101 outside of the pitch angle 6° and a tooth string 102 inside of the pitch angle 8° consisting of one member is disposed oppositely to the respective members 110 and 120, and the lock plate 100 is rotatable and movable in an axial direction. At the time of rotating the plate 100 at the four-fold speed of the rotation of the member 110, the two members 110 and 120 of the plate 100 are enabled to fit each other at every 2° change of the inclination of the bracket 112. When they are fitted with each other, the bracket 112 is locked to the bracket 122.