To avoid the curtailment of a period for overhauling by saving labor for maintenance while keeping an apparatus in a main motor clean without causing the intrusion of dust that is separated from the outside air through a filter and is prone to be accumulated at a dust discharging port.

Dust is automatically discharged to the outside together with an exhaust wind of a cooling fan through a dust discharging passage that reaches the vicinity of the cooling fan from the dust discharging port formed at the filter of the main motor installed on a bogie of a vehicle, and thereby the dust is prevented from being accumulated and intruding into the apparatus. By this, the main motor can be operated always under favorable conditions, the high performance of maintenance can be provided, and practical availability is enhanced.