A failure determining apparatus for a gas-insulated electrical appliance where the appliance is a cylindrical metal container with a charged conductor, insulating gas, and a plurality of insulating spaces inside for supporting the conductor and partitioning the container. The failure determining apparatus has a high-speed developing sensor which detects a high-speed phenomenon caused by a failure within the gas sections; a first failure determining circuit which determines the presence of a failure from an output of the high-speed developing sensor; a low-speed developing sensor which detects a low-speed phenomenon caused by a failure within the gas sections; a second failure determining circuit which determines the presence of a failure from an output of the low-speed developing sensor; and an arithmetic control circuit which identifies a gas section of the metal container in which a failure occurs from an output of the first and second failure determining circuits.