PURPOSE: To have a rigid and light seat slide by furnishing two or more lock detention parts at the base part of a reclining device and installing lock levers on the fixed rail side in such a way that each lever can be detained and released by its mating lock detention part.

CONSTITUTION: A seat back frame 1 and a seat cushion frame 2 are connected to a reclining device 3b while the base part 12b is secured to a moving rail 4. The end of the base part 12b is formed so as to mate with the side face 15 of the moving rail 4 and there two or more teeth 13b for locking are fixed. Teeth 13b for locking the levers 7b, capable of being detained and released freely, are mounted on the fixed rail 11 which is secured to the car body and supports the moving rail 4 slidably.