PURPOSE: To suppress a temperature rise inside a shielding box by a method wherein the air for cooling use is supplied to the inside of the shielding box in which an electronic circuit is housed and the air inside the shielding box is evacuated.

CONSTITUTION: The air inside a shielding box 1 flows into and is cooled at a radiator 7 via an air evacuation nozzle 4, a nonreturn valve 8, an air evacuation tube 9, an evacuation pump 10 and a pipe 11. The air which is cooled at the radiator 7 is returned to the shielding box 1 via an air supply tube 6, a nonreturn valve 5 and an air supply nozzle 3. Consequently, heat generated from an electronic circuit 2 which is housed inside the shielding box 1 can effectively be removed, and a temperature rise inside the shielding box 1 can be suppressed.