A tank filled with an insulating gas accommodates insulator tubes incorporating vacuum-valve breakers for individual phases. One end of each insulator tube is fixed to the inside of the tank. Each vacuum-valve breaker is installed generally on a common longitudinal axis with the relevant insulator tube in such a manner that its movable electrode rod is directed toward the fixed end of the insulator tube. The insulator tube has as its integral part a bus line fixing portion projecting in a direction intersecting the longitudinal axis of the insulator tube from the proximity of its end opposite to the fixed end to support a bus-side conductor in an insulated fashion.