An air circuit breaker comprising: a fixed element having a fixed contactor; a movable element having a movable contactor which is disposed so as to be contactable with and separable from the fixed contactor; an arc extinguishing chamber which is disposed upward of the fixed element and the movable element so as to be apart from the fixed element and the movable element and in which an arc generated upon opening between the fixed contactor and the movable contactor is divided and extinguished; a fixed-side arc runner which guides the arc to the arc extinguishing chamber and which has an end portion extending in a direction opposite to a direction toward the movable element, from the fixed element side between the fixed element and the arc extinguishing chamber; a movable-side arc runner which guides the arc to the arc extinguishing chamber and which has an end portion extending in a direction opposite to a direction toward the fixed element, from the movable element side between the movable element and the arc extinguishing chamber; characterized in that it comprises a mold frame enclosing the fixed element and the movable element beneath the arc extinguishing chamber and supporting the fixed element, the movable element, the fixed-side arc runner, and the movable-side arc runner, wherein the mold frame includes a fixed-side gas discharge passage through which a port provided in a side surface of the mold frame and a fixed-side gas discharge port provided in the mold frame so as to oppose the end portion of the fixed-side arc runner are in communication with each other and which is separated from the fixed element side by a fixed-side blocking wall which is a part of a wall enclosing the fixed element and the movable element, and a movable-side gas discharge passage through which a port provided in a side surface of the mold frame and a movable-side gas discharge port provided in the mold frame so as to oppose the end portion of the movable-side arc runner are in communication with each other and which is separated from the movable element side by a movable-side blocking wall which is a part of the wall enclosing the fixed element and the movable element. a fixed element having a fixed contactor; a movable element having a movable contactor which is disposed so as to be contactable with and separable from the fixed contactor; an arc extinguishing chamber which is disposed upward of the fixed element and the movable element so as to be apart from the fixed element and the movable element and in which an arc generated upon opening between the fixed contactor and the movable contactor is divided and extinguished; a fixed-side arc runner which guides the arc to the arc extinguishing chamber and which has an end portion extending in a direction opposite to a direction toward the movable element, from the fixed element side between the fixed element and the arc extinguishing chamber; a movable-side arc runner which guides the arc to the arc extinguishing chamber and which has an end portion extending in a direction opposite to a direction toward the fixed element, from the movable element side between the movable element and the arc extinguishing chamber; characterized in that it comprises a mold frame enclosing the fixed element and the movable element beneath the arc extinguishing chamber and supporting the fixed element, the movable element, the fixed-side arc runner, and the movable-side arc runner, wherein the mold frame includes a fixed-side gas discharge passage through which a port provided in a side surface of the mold frame and a fixed-side gas discharge port provided in the mold frame so as to oppose the end portion of the fixed-side arc runner are in communication with each other and which is separated from the fixed element side by a fixed-side blocking wall which is a part of a wall enclosing the fixed element and the movable element, and a movable-side gas discharge passage through which a port provided in a side surface of the mold frame and a movable-side gas discharge port provided in the mold frame so as to oppose the end portion of the movable-side arc runner are in communication with each other and which is separated from the movable element side by a movable-side blocking wall which is a part of the wall enclosing the fixed element and the movable element. a fixed-side gas discharge passage through which a port provided in a side surface of the mold frame and a fixed-side gas discharge port provided in the mold frame so as to oppose the end portion of the fixed-side arc runner are in communication with each other and which is separated from the fixed element side by a fixed-side blocking wall which is a part of a wall enclosing the fixed element and the movable element, and a movable-side gas discharge passage through which a port provided in a side surface of the mold frame and a movable-side gas discharge port provided in the mold frame so as to oppose the end portion of the movable-side arc runner are in communication with each other and which is separated from the movable element side by a movable-side blocking wall which is a part of the wall enclosing the fixed element and the movable element.