A gas circuit breaker comprising: a fixed arc contact extending along an operating axis; a moving arc contact allowed to move to a position where the moving arc contact contacts the fixed arc contact and a position where the moving arc contact is separated from the fixed arc contact by moving along the operating axis; a frame surrounding a circumference of the moving arc contact to form a puffer chamber around the moving arc contact; a nozzle having a cylindrical shape centered on the operating axis, the nozzle being fixed to the frame and projecting in a direction toward the fixed arc contact from the moving arc contact; and an arc extinction assisting portion provided on an inner surface of the nozzle and made of an ablation material, wherein the nozzle includes a fixed portion having a cylindrical shape centered on the operating axis and fixed to the frame, and a throat portion having a cylindrical shape centered on the operating axis and extending from the fixed portion toward a side where the fixed arc contact is present along the operating axis, wherein an inner diameter of the throat portion is smaller than an inner diameter of the fixed portion, and wherein the arc extinction assisting portion forms a connecting surface that connects an inner surface of the throat portion to an inner surface of the fixed portion, characterized in that the nozzle and the arc extinction assisting portion are provided with a fall-off preventing portion to prevent the arc extinction assisting portion from falling off the nozzle, and wherein the fall-off preventing portion includes a recessed portion formed on the throat portion and recessed in a direction along the operating axis, and a projecting portion of the arc extinction assisting portion, the projecting portion being fitted into the recessed portion. a fixed arc contact extending along an operating axis; a moving arc contact allowed to move to a position where the moving arc contact contacts the fixed arc contact and a position where the moving arc contact is separated from the fixed arc contact by moving along the operating axis; a frame surrounding a circumference of the moving arc contact to form a puffer chamber around the moving arc contact; a nozzle having a cylindrical shape centered on the operating axis, the nozzle being fixed to the frame and projecting in a direction toward the fixed arc contact from the moving arc contact; and an arc extinction assisting portion provided on an inner surface of the nozzle and made of an ablation material, wherein the nozzle includes a fixed portion having a cylindrical shape centered on the operating axis and fixed to the frame, and a throat portion having a cylindrical shape centered on the operating axis and extending from the fixed portion toward a side where the fixed arc contact is present along the operating axis, a fixed portion having a cylindrical shape centered on the operating axis and fixed to the frame, and a throat portion having a cylindrical shape centered on the operating axis and extending from the fixed portion toward a side where the fixed arc contact is present along the operating axis, wherein an inner diameter of the throat portion is smaller than an inner diameter of the fixed portion, and wherein the arc extinction assisting portion forms a connecting surface that connects an inner surface of the throat portion to an inner surface of the fixed portion, characterized in that the nozzle and the arc extinction assisting portion are provided with a fall-off preventing portion to prevent the arc extinction assisting portion from falling off the nozzle, and wherein the fall-off preventing portion includes a recessed portion formed on the throat portion and recessed in a direction along the operating axis, and a projecting portion of the arc extinction assisting portion, the projecting portion being fitted into the recessed portion. a recessed portion formed on the throat portion and recessed in a direction along the operating axis, and a projecting portion of the arc extinction assisting portion, the projecting portion being fitted into the recessed portion.