PURPOSE: To enable to cool a power transistor and to simply mount or dismount the transistor while a heat pipe is mounted in a method of cooling the power semiconductor used for a power source circuit mainly for power transistor, thyristor by inserting, sealing one end of the pipe to the cap and disposing the other end side in a duct, in which cooling air flows.

CONSTITUTION: One end A of a heat pipe which is slightly larger in diameter than a cap 2 is sealed by a transistor 1, and the other end B is sealed ordinarily by a heat pipe forming material. A heat resistant plastic is sealed between the base 3 contacted with a wick and the tubular wall of the pipe. A hole 11 which is slightly larger in diameter than the pipe 5 is formed at the tubular wall of a duct 9 in which cooling air flows in a direction designated by broken lines by the operation of a fan 10, part of the pipe 5 is disposed in the duct 9 through the hole, and a condenser is cooled by air which flows through the duct.