The invention relates to an apparatus for drawing off ground air under the foundation slab of a house. A tube is placed in an excavation made some meters away from the house. The bottom end part of the tube is downwardly open and is provided with a plurality of rows of holes through which a blower, arranged inside the pipe, draws air from the ground. The ground air which is withdrawn departs via an evacuation duct opening out above the ground surface. A lid is arranged airtight at the upper end of the tube. The evacuation duct connects up with the tube in an area above the blower.