The present invention relates to a puffer type circuit breaker having a high voltage and large capacity, which is provided with an insulating nozzle for blowing a arc-extinguishing gas of high pressure generated by separating a movable contactor from a fixed contactor. The fixed contactor has a larger diameter portion formed on the base portion thereof and a smaller diameter portion formed at the tip end thereof, and the insulating portion has a first throat portion adjusted by the smaller diameter portion and a second throat portion located at the downstream side of the first throat portion and adjusted by the larger diameter portion, thereby allowing the gas inlet portion to be formed by the diameter difference between the first throat portion and the smaller diameter portion which is in such manner that the second throat portion is adjusted by the larger diameter portion. By virtue of these features, when the contactors are separated from each other, even if the arc-extinguishing gas of high pressure flows into the space between the first throat portion and the second throat portion, the rapid gas flow is not generated therein. Accordingly, since the arc-extinguishing gas is prevented from lowering the gas density on the tip of the fixed contactor, it is possible to improve the breaking ability of small capacitive current.