PURPOSE: To prevent a polluting material from scattering to other section even when a door is opened, by putting a breeding box into a duct and changing air speed in the duct according to the position of the breeding box.

CONSTITUTION: A breeding box 21 is arranged in the inner part of a duct 22 in breeding and an opening part 22a is closed and air 33 sucked into the dust by a blower 40 is allowed to flow from the lower face of a door 11 into the breeding box and discharged through a filter 20. Air is allowed to flow out also from the opening part 22a to prevent lowering of inflow air speed, as opening area of the opening part 22a is increased by pulling out the breeding box 21 on this side.