PURPOSE: To increase current carrying capacity with a movable part, becoming a maximum temperature part, cooled by providing heat radiation blocks on a clamp through which the movable steam of a vacuum valve passes.

CONSTITUTION: Plural heat radiation blocks 14 are fastened with plural bolts respectively on either side and the front side of a clamp 6, and plural cooling fins are formed on these heat radiation blocks 14. A cooling wind, passing through upward between a lower part conductor 4B or a vacuum valve 5 and a barrier 11 from the introduction part 10 of the front lower part via an opening part 2a1, rises from the front and rear and the right and left of the lower part conductor 4B through among the cooling fins of the heat radiation blocks 14 fitted to the outer periphery of a clamp 6 on the upper part of the lower part conductor 4B, and consequently the heat of a movable stem 5b is quickly cooled via the clamp 6. This permits improvement of the heat radiation effect and restraint of the temperature rise of the movable stem 5b, obtaining a vacuum circuit breaker able to increase current carrying capacity.