PURPOSE: To cool a component at the side near a surface unit cover of a package by providing a cover having a suction main opening having large area at a lower part and a suction auxiliary opening having an area smaller than that of the main opening at an upper part on the front face of a book shelf unit.

CONSTITUTION: When an electronic device is mounted in the installing environment of an air conditioning system in which the flowing air of a natural cooling system is diffused from an air conditioning hole formed at a floor panel into a room and sucked from a ceiling or a sidewall, the air is sucked from a sucking main opening 16 formed at the lower part of a front unit cover 18 provided on the front face of a case 14 for containing a book shelf unit 11 and having a large area, and a suction auxiliary opening 17 formed at an upper part, the air which removes heats from electronic components, such as IC placed on a package 13 flows along a convection guide plate 12A, and discharges from the outlet 12B of a convention guide plate structure 12. Accordingly, the component near the cover 15 is also cooled to efficiently cool the package 13.