The front case of the unit case contains a circuit section including a power semiconductor module, and the rear case contains a cooling fin assembly of the semiconductor module and a cooling fan. The cooling air ventilated through the wind channel by the cooling fan is discharged upward from the unit case. The water-proof cover is placed over the top of the unit case. The water-proof cover includes an exhausting structure formed with openings downward at the left and right sides of a front part of the water-proof cover to form an exhausting path; and a ventilation guide formed around an exhausting port inside a rear part of the water-proof cover. The cooling air discharged to the inside space of the water-proof cover by the cooling fan is ventilated through the ventilating path and exhausted outside through the exhausting path.