Disclosed is an automatic locking apparatus used in a guide rail for a drawer. The automatic locking apparatus is provided at an end portion of a fixing slider to lock a movable slider when the drawer is closed. The automatic locking apparatus prevents the drawer from being opened and smoothly locks the drawer. A stopper ring is attached to the movable slider, and the automatic locking apparatus is provided on the fixing slider to cooperate with the stopper ring. The automatic locking apparatus includes a fixing plate attached to the fixing slider, a body fixed to the fixing plate while protruding toward the movable slider, an elastic member having a substantially “C” shape and being rotatably coupled to one end of the body, and a rotating member having a substantially “reverse-L” shape and being rotatably coupled to the other end of the body.