A main circuit switching unit and an isolator are accommodated in a vacuum container. A first conductor and a main circuit conductor are arranged to extend in and out of the vacuum container. A movable electrode of the main circuit switching unit is connected to the first conductor through a first flexible conductor, and a movable electrode of the isolator is connected to the main circuit conductor through a second flexible conductor. A shield is arranged around the first conductor and the main circuit conductor. Another shield is arranged around a connecting portion between the first flexible conductor and the movable electrode of the main circuit switching unit. A still another shield is arranged around the isolator. A further another shield is arranged around the second flexible conductor. These shields are secured to the vacuum container through insulating spacers. This arrangement instantly eliminates a dielectric breakdown phenomenon that occurs at electric field concentrated regions by the shields to prevent a dielectric breakdown caused by particulate foreign matters and thereby improve an insulation reliability of the devices.