PURPOSE: To ventilate a small room while operating a fan at its full capability and to ventilate a large room efficiently in accordance with the capability of the fan, by introducing outside air into the fan at the time of ventilating the small room.

CONSTITUTION: Shutters 8, 9 provided at ventilating ports 5, 6 are operated by switches 8a, 9a provided in the vicinity of the shutters 8, 9, respectively. On the outside of a building 1 is provided a fan 10 which communicates with a ventilating duct 7. The fan 10 is of a rated capacity type, and has a rated capacity set in conformity with the internal volume of a room 2. Therefore, when the large-volume room 2 is ventilated, proper ventilation is achieved. For ventilation of a small room 3, an outside air intake duct 12 communicating with an outside air inlet 11 at one end and with the ventilating duct 7 at the other end is provided on the outside of the building 1, and the duct 12 is equipped with a shutter 14.