PURPOSE: To enable stable ventilation always, by controlling a revolution of electric motor so as to keep air pressure of an exhaust duct or an air supply duct constant through a signal from an air pressure sensor.

CONSTITUTION: A blade 1 for discharge of air is revolved by an electric motor 2 and an air pressure sensor 5 is provided in an exhaust duct 4 or an air supply duct 3 to the blade 1. A control part 6 for control of a revolution of the electric motor 2 through a signal from the air pressure sensor 5 is provided. At the control part 6, the revolution of the electric motor 2 is controlled so as to keep air pressure of the exhaust duct 4 or the air supply duct 3 constant always through the air pressure sensor 5. With this construction, irrespective of a difference of pressure losses to be applied to the ventilating fan, a stabilized and fixed quantity of ventilation is discharged always and stabilized ventilation can be performed alwasy.