The present disclosure provides a locking device for the precise adjustment of the movement position of a vehicle seat rail relative to a fixed rail while not compromising the strength of the seat rail. The locking device includes the seat rail having locking holes, and four locking pins to be inserted in the locking holes. In particular, the locking holes and pins are arranged with predetermined gaps along a longitudinal direction of the seat rail, and a distance between second and third locking pins is different from a distance between first and second locking pins which is the same as a distance between third and fourth locking pins.