A puffer type gas-blast circuit breaker provided with a movable electrode and a fixed electrode opposite the movable electrode, which electrodes are separable for implementing a circuit-breaking action, and a fixed current-carrying contact arranged around the periphery of the fixed electrode. The movable electrode is equipped with a surrounding insulating nozzle having a tapered inside surface. Movement of the movable electrode during separation of the electrodes compresses the gas in a puffer chamber and so blows out the resulting arc between the movable and fixed electrodes. The circuit breaker further includes a cylindrical insulator or capacitor that surrounds the arc extinction chamber formed by the electrodes and the insulating nozzle between the movable and fixed electrodes. In the circuit-breaking action, the line of extension of the tapered inside surface of the insulating nozzle downstream, as regards the gas flow, from the throat portion of the insulating nozzle, and extending in the direction of the fixed contact, lies within the innermost portion of the extreme end portion of the fixed current-carrying contact, on the side of the fixed contact nearest the movable electrode.