A gas circuit breaker comprising a pair of contactors contact portions of which are separable relatively from each other, an insulating nozzle of an electrically insulating material surrounding the contact portions of said contactors so as to guide a flow of gas, and a puffer chamber for compressing the gas in conjunction with a separating operation of the contact portions so as to supply it under guidance of the insulating nozzle, the gas from the puffer chamber being exhausted through exhaust passages passing through a hollow portion of the one contactor located within the insulating nozzle, wherein the exhaust passages are formed between the puffer chamber and the one contactor, and the gas circuit breaker further comprises block means serving to close during an initial stage of the separating operation and open afterward exhaust ports formed at ends of the exhaust passages located on the downstream side of the gas flow. This makes it possible to reduce the flow resistance to the gas flow used for arc extinguishment in cooperation with an arc-extinguishing gas flow passing through a throat portion of the insulating nozzle as well as to reduce a force required for operation.