To eliminate the necessity for keeping gas pressure for a long time and make an operation mechanism small by dispatching an opening instruction in such timing that current to be shut off comes zero point in a period in which arc extinguishing gas pressure in a compressing space is increased more than a threshold value previously set according to the kinds of gas.
In opening operation, an operation rod 21 moves together with a moving contact part 20, and when a fixed arc contact 11 and a moving arc contact 23 are separated, an arc 31a is generated. Since a fixed piston 26 is fixed to the inside of a container, arc extinguishing gas in a compressing space S1 formed with the fixed piston 26, the operation rod 21, and a moving cylinder 22 is mechanically compressed to increase pressure. In the state the pressure in the compressing space S1 is sufficiently increased, when the throat of an insulating nozzle 25 is opened, gas flows toward the fixed arc contact 11 through a gas flow path 22a between the moving arc contact 23 and the nozzle 25 to conduct arc extinguishing. An opening instruction is dispatched according to the wave form of fault current detected with a current detector.