| 摘要 |
PROBLEM TO BE SOLVED: To secure a cooling effect in the same manner when one step is mounted without using a cooling fan by a method wherein there is provided switching means for switching an airflow route between a first state and a second state in a lower part of one board unit out of a plurality of the board units. SOLUTION: Board units 12 of a multi-step are mounted vertically in a unit accommodation part 11a in a casing 11, and these board units 12 are provided with a plurality of circuit boards 13 mounting ICs, etc., respectively. Furthermore, each board unit 12 can pass an air from downward to upward through between the adjacent circuit boards 13. There is provided switching means 14 in a lower part of each board unit 12, and the switching means 14 switches an airflow route between a first state that the air from lower part is flown to the board unit 12 and a second state that the air from lower part is intercepted and the air from a vacant space part 11d is flown to the board unit 12.
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