PURPOSE: To improve the operability and operate smoothly by sliding one end of two link members in long holes provided on a rotary member and a stationary member respectively.

CONSTITUTION: One end of a rotary member 32 fixed to a seat frame is pivoted on one end of a stationary member 31. One end of the first link member 33 is pivoted on the lower end of the rotary member 32, and one end of the second link member 44 is pivoted on the middle of the first link member 33. A lock pin 48 fitted to the other end of the second link member 44 is inserted into a long hole 49 of the rotary member 32, and a lock plate 50 with a plurality of grooves to be coupled with the lock pin 48 is fitted to the rotary member 33. A pin 40 to be fitted to the other end of the first link member 33 is inserted into a long hole 38. The lock pin 48 and the pin 40 at the other end of the first link member 33 slide in the long holes 49, 38, and the lock pin 48 is selectively coupled with a groove of the lock plate 50, thus adjusting the seat height.