To prevent, using small electric energy, dew condensation at an exhaust duct that cause corrosion and odors.

Air fed by a lower fan 5 is warmed by a heater 16 and blown as hot air into a bathroom to dry the washing. During a drying operation, humidity rises because of moisture vaporized from the washing, and this high- humidity air is exhausted outside through an exhaust duct 17. A humidity sensor 21 mounted inside the exhaust duct 17 then measures humidity at the exhaust duct 17, and when the humidity exceeds 50%, an electric power of 50W is supplied to a cord heater 20. Thereafter, as the humidity rises the electric energy supplied to the cord heater 20 is increased and the exhaust duct 17 is heated by the cord heater 20 to prevent dew condensation at the exhaust duct 17.