Abstract: The object of this invention is to provide an automatic container locking apparatus for trailers. This locking apparatus is automatically operable in response to the weight of a container to lock or unlock the container on the deck of a trailer. This locking apparatus has a housing (10) vertically set in the deck (1), with a spring-biased actuator (20) movably set within the housing (10). A spring-biased locking bolt (30) having a top head (31) is movably set within the actuator (20). This locking bolt (30) has a stopper (32) and a curved guide slit (41), thus being selectively moved together with the actuator (20) while being rotated. A support shaft (50) holds the vertical position of the locking bolt (30) within the housing (10). An actuation pin (51) of the shaft (50) engages with the curved guide slit (41), thus allowing the locking bolt (30) to be rotatable when the locking bolt is vertically moved together with the actuator (20).