A low-voltage power source circuit board is provided downstream in relation to the flow of air introduced through an intake hole and exhausted by a power source fan. Heat sinks are provided on the low-voltage power source circuit board on electronic parts that generate a large amount of heat, the plate surfaces of the heat sinks being arranged along the direction of airflow, thereby minimizing their resistance to the airflow. Air including heat generated from these electronic parts is exhausted by the power source fan without the air heating the high-voltage power source circuit board or engine circuit board that are disposed upstream in relation to the direction of airflow.