PURPOSE: To set an operating time of ventilation fan to a time corresponding to a condition of a bath room to be used by a method wherein an automatic ventilation device for the bath room is comprised of a dew formation sensor for sensing a humidity in the bath room, a control part for determining a ventilation fan operating time and a switch capable of changing the operating time.

CONSTITUTION: In the event that a user turns on a power supply switch 5 and a mode changing-over switch 6 placed out of a bath room and an automatic operation is set, a signal of a dew formation sensor 1 corresponding to a predetermined reference humidity is sent to a micro-computer, a signal is transmitted to a triac through the micro-computer and then a ceiling buried ventilation fan 4 is operated. The micro-computer acknowledges a rate of variation from a time when a signal of the dew formation sensor is dropped, and an operating time of the ventilation fan corresponding to the rate of variation. This is also changed in response to a set state of the changing-over switch 6.