PURPOSE: To provide a controller for a ventilation fan which adjusts the output of a single phase induction motor which drives blades in accordance with airtightness degree in a room where a ventilation fan is installed and reduces the noise of the ventilation fan and minimizes its energy loss.

CONSTITUTION: There are installed a current detection circuit 9 which detects the output of a current detection device 8 which generates electromotive force in response to the electric current of a single phase induction motor, an output holding means 11 which holds the output of the single phase induction motor when the current detection device 8 detects the electric current which exceeds a setting value after the single phase induction motor is powered on and a specified time has elapsed and an output increase means 12 which raises the output of the single phase induction motor when the current detection device 8 detects currents less than the setting value.