While one of ventilation fans operates, the other ventilation fan is at rest. Air flows from the operating ventilation fan to the ventilation fan at rest. The airflow passes through an air passage defined in a heat radiating member located between the ventilation fans. Dust cumulates at the entrance of the air passage. The other ventilation fan is then driven to operate while the operating ventilation fan is brought to rest. Airflow is generated in the reverse direction. The airflow is inverted within the air passage. The inverted airflow serves to remove the dust from the entrance of the air passage. The cumulation of the dust can thus reliably be avoided. Accordingly, no dust filter is required in the path of airflow outside the air passage. Airflow of a sufficient amount can always be maintained in the air passage.