PURPOSE: To aim at stabilization of the temperature of cooling wind, while introducing the outside air for cooling by supplying a part of exhaust air coming out of a forced air cooling type electronic tube into an inlet port of the electronic tube together with the outside air recirculatingly by means of another blower.

CONSTITUTION: The first air pipe 23 opened to the outside air and the third air pipe 25 are connected to an exhaust port of a forced air cooling type electronic tube 6 through the first blower 21, and the other end of the air pipe 25 is connected to the middle of the second air pipe 24 connecting the inlet port of the electronic tube 6 to the outside air. Recirculating cooling is performed through the second blower 22, and a recirculating air quantity is regulated by regulating a rotational speed of the second blower 22. As the recirculating air quantity can be varied over a wide range, it can follow a wide range variation of the outside ambient temperature. As the recirculating cooling air is pressurized by the second blower, the air pipe can be made in a small size.