A locking apparatus of a buckle of a seat belt for vehicles includes a sector gear bracket configured to be integrally coupled to a rear pipe which is coupled to a seat cushion frame, wherein the sector gear bracket has: an arc-shaped locking hole; and a sector gear arranged along the locking hole as an internal gear; a pole bracket located at one side of the sector gear bracket, wherein the pole bracket has a shaft part which is configured to pass through the locking hole and the seat cushion frame and then be integrally coupled to a lower end of the buckle; and a lock gear arranged on the pole bracket as an external gear.