Sealed chamber for monitoring pressure 23 is provided on sealed tank 37. Sealed tank 37 is filled with insulating gas. And, a connection using insulated pipe 19 is provided between medium-pressure space 18 that makes a medium pressure and sealed chamber for monitoring pressure 23. In sealed chamber for monitoring pressure 23, a pressure monitoring and controlling means is provided for monitoring the internal pressure thereof to activate the pressure-release control in response to the situation where the monitored internal pressure has reached the predetermined setting value. The pressure monitoring and controlling means includes solenoid valve 28 that is capable of opening sealed chamber for monitoring pressure 23 to the atmosphere, pressure detector 26 that detects the internal pressure, and pressure monitor 27, which activates solenoid valve 28 for pressure-release when pressure detector 26 detects that the internal pressure has reached the predetermined abnormal pressure value.