To use a gas-blast circuit breaker to the utmost limit of its usage limit by finding gas pressure corresponding to a consumption of an arc contact.

Gas pressure is detected all the time by a pressure sensor 1 mounted inside a gas-blast circuit breaker, and a pressure signal is fed to a processing part 6 via an input processing part 4 and an A/D converting part 5. When the gas-blast circuit breaker is operated, a cut-out signal is fed to the processing part 6 from an auxiliary switch 2 interlocked with the operation via a contact point input part 8. The processing part 6 calculates a gas pressure increase value based on change of the pressure signal, using the cutout signal as trigger, to be stored in a memory, and determines whether an accumulated value thereof reaches to a life value preset in the memory or not When the processing hereinbefore is repeated in every cutout to reach to the life value, a result is output to an external output part 7. A life of an arc contact is monitored by this manner, and the gas-blast circuit breaker is used to the utmost limit of its usage limit.