[en] To conduct stable tritium breeding by always maintaining a required temperature difference between coolant pipeways through whch coolants are circulated and tritium breeding material. That is, outer pipeways are disposed at a gap to the outer circumference of cooling pipeways for cooling the blanket. Wires are wound around spirally along the outer circumference of the cooling pipeways for surely maintaining the gap between the outer pipeways and cooling pipeways. Accordingly, it is possible to surely maintain a required temperature difference between the cooling pipeways and the breeding material. As a result, it is possible to stabilize the tritium breeding. (I.S.)