A door locking device of a distribution board of an air circuit breaker, including a transmission shaft transmitting power for drawing in and out a circuit breaker body, a shaft link rotatably coupled to one end of the transmission shaft, a link rod having one end rotatably coupled to a portion of the shaft link, a cam plate rotatably coupled to the other end of the link rod and pushing or pulling the circuit breaker boy, and a constraint plate installed in a distribution board door, further includes a pressing part protruding from a portion of the link road, and a hook plate rotatably coupled to a side of the cradle and having one end in contact with the pressing part to receive rotational force and the other end restricting or releasing the constraint plate, wherein the hook plate releases the constraint plate in a disconnection position.