PURPOSE: To prevent intrusion of dust and radiation of a disturbing radio wave by arranging 1st and 2nd slide plates slid along with rails to an opening part so as to be opposite to each other in the lengthwise direction of the opening part and opening/closing the opening part with sliding of the plates.

CONSTITUTION: First and second slide plates 21, 22 to be slid are arranged to an opening part 20 so as to be opposite to each other in the lengthwise direction of the opening part and the 1st and 2nd slide plates 21, 22 are slid in the direction of the arrow E by the operation of knobs 21A, 22A respectively to open/close the operating part 20. For example, when a I/O card 4 is inserted to the processing unit, the opening part 20 is closed by the slide plates 21, 22 and the opening area of the opening part 20 is reduced to the utmost, resulting in reducing the gap. Furthermore, 1st and 2nd turning plates to open/ close the opening part 20 with the turning are provided to the opening part 20 so as to be opposite to each other in the short-side direction of the opening part 20. Then a spring pressing the 1st and 2nd turning plates in the closing state is provided to open the opening part 20 when the I/O card 4 is inserted to the opening part 20.