In this invention, the CPU of a NAS board, which is liable to become hot, is cooled by concentrating a larger amount of air onto a heat sink. A plurality of logic circuit boards 3 are mounted on a control section 1D of a disc array device 1. The circuit component 4A for realizing the NAS function, liable to become hot, is provided with a heat sink 5. The aperture of the main air inlet section (Wb) is covered with a top plate 8. Airflow guidance sections (Wa, Wc) are respectively provided on both sides of the main air inlet section. Since the fin pitch of the airflow guidance sections is set narrow, the air inflow resistance (airflow resistance) of these is higher than that of the main air inlet section. Due to this difference in airflow resistance, the air in the vicinity of the airflow guidance sections is guided into the main air inlet section. The top plate 8 prevents outflow of air that has flowed into the main air inlet section to outside the heat sink 5.