PURPOSE: To save energy by arranging an air feed cylinder on the circumference of an exhaust cylinder projecting outdoors to be connected to an air feed blower with the exhaust cylinder to an exhaust blower while the discharge port of an air feed duct is connected to another air feed cylinder with a damper in a switchable manner.

CONSTITUTION: A contaminated warm air in the room is sucked with an exhaust blower 2 and discharged 9 from an exhaust cylinder 7. On the other hand, a fresh outdoor air 10 is sucked with an air feed blower 4 and fed into the room via an air feed cylinder 8 and a duct 12. During the period, it undergoes a heat exchange with the warm air in the exhaust cylinder 7 and turns to a fresh warm air. In this case, a damper 18 is opened at the position as indicated by the solid line. In the summer season or the like, the damper 18 is set to the position as indicated by the dotted line with the duct 12 closed so that a cool fresh outdoor air is taken in from an air feed cylinder 15 whereby energy can be saved.