PURPOSE: To avoid the dropping of water droplets onto a user of a bathroom during his taking a bath or the like, and to prevent the main body of the title ventilating fan and a ceiling material from being corroded and rotted by opening and closing the freely openable bottom of the lowermost recessed part of a louver in response to an output signal produced from a controlling circuit which is mounted on the indoor side of the louver and which converts a detection signal by a infrared ray sensor for detecting a human body.

CONSTITUTION: Since the recessed part 9 of a louver 6 is slowly inclined toward the central part thereof, water droplets run toward its central part and are collected to a part where a bottom 10 is formed. When there is a person in a bathroom, a detection signal is produced from an infrared ray sensor 14, and a controlling circuit 15 converts this detection signal to pull a drive means 13 upward. Thereupon, since the bottom 10 is mounted at the top end of the drive means 13, the bottom 10 is to close the recessed part 9 and, as a result, condensed dew droplets collected into the recessed part 9 do not drop down into the bathroom. Moreover, the louver 6 having the recessed part 9 formed by uprightly providing a rib 8 covering the entirely circumferential part of a vent port 7 is provided, at its recessed part 9, with a controlling circuit 19 which closes a power circuit for a heater 17 upon receipt of a detection signal from a dew condensation sensor 16 for detecting dew condensation and which is fixed to the upper part of the rib 8.