An air circuit breaker is disclosed. An air circuit breaker according to one aspect of the present disclosure includes a cradle and a circuit breaker main body drawn-in and drawn-out to/from the cradle; and further includes: an interlock lever connected to an off button of the circuit breaker main body to move vertically; and a transfer device unit provided on the lower side of the cradle, wherein the transfer device unit can include a key interlock assembly, which is provided to be horizontally movable so that, in a state of having moved toward one side thereof, the movement of the interlock lever is restricted and, in a state of having moved toward the other side thereof, the movement of the interlock lever is permitted.