A vacuum circuit breaker comprising: a grounded tank hermetically containing an insulating gas, the grounded tank being tubular; a vacuum valve insulated and supported inside the grounded tank, the vacuum valve including a movable contact and a stationary contact; a movable conductor electrically connected to the movable contact; a stationary conductor electrically connected to the stationary contact; a movable-side external conductor and a stationary-side external conductor disposed in a pair of bushings extending upward from the grounded tank; a movable-side frame electrically connecting a lower end of the movable-side external conductor to the movable conductor; a stationary-side frame electrically connecting a lower end of the stationary-side external conductor to the stationary conductor; an insulating operation rod connected to an operating device disposed outside the grounded tank to operate the movable contact; a movable-side insulating support tube to support the movable-side frame and insulates thereof from the grounded tank; a stationary-side insulating support tube to support the stationary-side frame and insulates thereof frame from the grounded tank; a movable-side flange covering a movable-side end of the grounded tank wherein an arc-extinguishing chamber is formed by connecting: the movable-side insulating support tube; the movable-side frame; the vacuum valve; the stationary-side frame; and the stationary-side insulating support tube, and the arc-extinguishing chamber is housed in the grounded tank, characterized in that the movable-side flange includes a hole allowing passage of the movable-side insulating support tube; and a flexible member (17, 18) on an inner peripheral surface of the movable-side flange, wherein the movable-side insulating support tube is supported, at an end opposite from an end connected to the movable-side frame, by the movable-side flange via the flexible member (17, 18) and is movable along an axial direction and a radial direction of the arc-extinguishing chamber. a grounded tank hermetically containing an insulating gas, the grounded tank being tubular; a vacuum valve insulated and supported inside the grounded tank, the vacuum valve including a movable contact and a stationary contact; a movable conductor electrically connected to the movable contact; a stationary conductor electrically connected to the stationary contact; a movable-side external conductor and a stationary-side external conductor disposed in a pair of bushings extending upward from the grounded tank; a movable-side frame electrically connecting a lower end of the movable-side external conductor to the movable conductor; a stationary-side frame electrically connecting a lower end of the stationary-side external conductor to the stationary conductor; an insulating operation rod connected to an operating device disposed outside the grounded tank to operate the movable contact; a movable-side insulating support tube to support the movable-side frame and insulates thereof from the grounded tank; a stationary-side insulating support tube to support the stationary-side frame and insulates thereof frame from the grounded tank; a movable-side flange covering a movable-side end of the grounded tank wherein an arc-extinguishing chamber is formed by connecting: the movable-side insulating support tube; the movable-side frame; the vacuum valve; the stationary-side frame; and the stationary-side insulating support tube, and the arc-extinguishing chamber is housed in the grounded tank, characterized in that the movable-side flange includes a hole allowing passage of the movable-side insulating support tube; and a flexible member (17, 18) on an inner peripheral surface of the movable-side flange, wherein the movable-side insulating support tube; the movable-side frame; the vacuum valve; the stationary-side frame; and the stationary-side insulating support tube, and the arc-extinguishing chamber is housed in the grounded tank, characterized in that the movable-side flange includes a hole allowing passage of the movable-side insulating support tube; and a flexible member (17, 18) on an inner peripheral surface of the movable-side flange, wherein the movable-side insulating support tube is supported, at an end opposite from an end connected to the movable-side frame, by the movable-side flange via the flexible member (17, 18) and is movable along an axial direction and a radial direction of the arc-extinguishing chamber.