PURPOSE: To prevent deposit of snow to a filter surface, by a method wherein a bent air duct is located between an open air intake port and a filter, and the intake open air is forced into collision with a wall surface, positioned facing the open air intake port, to remove snow contained in the open air.

CONSTITUTION: With an air feed processing device 3 started during snowfall, snow is taken in a building 1 through an open air intake port 1a together with the open air. The open air accompanied by snow is guided through the open air intake port 1a to an air duct 20, where a most snow in the open air is collided with an open air collision wall 24 of an inlet part 20a by an inertia force to be deposited to the wall for removal. Snow, adhered o the open air collision wall 24 and dropped to a floor, melts to produce molten snow water, which is drained through a drain system 25 in a building. Remaining snow is dropped for removal during passage of the open air through a horizontal part 20b of the air duct 20. After the flow of the open air is bent in an inverted- C shape at a terminal part 20c of the air duct 20, the open air is guided to an air filter 4, and when the open air flows in a bent manner, snow remaining in the open air is removed by an inertia force.