A tank type gas circuit breaker for a three-phase alternating current, comprises: three hollow members spaced in parallel with each other; a single stationary side end plate air-tightly fixed to first ends of the three hollow members; a single mechanism side end plate fixed to second ends of the three hollow members; three stationary contacts each supported to the stationary side end plate and extending in the three hollow members; three movable contacts each supported to the mechanism side end plate and extending in the three hollow members; insulated rods extending in the three hollow members while passing through the mechanism side end plate for interrupting contacts between the movable contacts and the stationary contacts; an operational rotary shaft mechanism disposed adjacent to the mechanism side end plate and connected to the insulated rods for driving each of the insulated rods; an actuator for driving the operational rotary shaft mechanism; two bushing porcelain insulators located on a top surface of each of the hollow members; current transformers arranged in lower sides of the bushing porcelain insulators; conductors disposed within the bushing porcelain insulators and connected to the movable and stationary contacts, and arc-extinction gas filled in the hollow members.