To improve accuracy in detecting a minute ground fault, by using a simple means for removing an influence of a gas pressure variation caused by an abrupt change in weather like rain fall.

In a gas insulation apparatus 1, a gas pressure sensor 4 for measuring gas pressure is provided for each gas section separated by a spacer 3. A variation in gas pressure for each gas section is calculated by a gas pressure variation calculating circuit 11. An open air temperature sensor 5, a rainfall sensor 6, and a sunshine sensor 7 are provided in the atmosphere outside the gas insulation apparatus 1. Then, variations in open air temperature, and amount of rainfall and sunshine are calculated by each calculating circuits 12 to 14. An influence of the variations in open air temperature, rainfall, and sunshine is calculated by a gas pressure variation compensating circuit 15, and these values are subtracted from each gas pressure variation. A compensated value of gas pressure variation is converted into a value at a reference temperature so that a gas section with a pressure variation larger than a threshold value is regarded as a failure point by a failure point orientation circuit 17.