PURPOSE: To make it possible to efficiently radiate heat generated in thyristor elements to the outside without using a large cooler by attaching heat radiation fins to the outside of a box to house the thyristor elements and joining the thyristor elements and the heat radiation fins with electrically insulated heat pipes.

CONSTITUTION: Metal blocks 3 of high-heat-conductivity metal, such as copper, are stuck to both sides of thyristor elements 2 and the thyristor elements 2 in a box 1 and beat radiation fins 4 outside the box 1 are joined with electrically insulated heat pipes 5, therefore, heat generated in the thyristor elements 2 can be guided to the heat radiation fins 4 about one thousand times as quickly as normal solid heat conduction. Hence the surface area of the heat radiation fins 4 is reduced and sufficient cooling is made possible without a cooling fan for forced cooling because the temperature of the heat radiation fins 4 can be made approximately the same as that of the thyristor elements 2.