The humidity detecting device includes a detection unit and a controller. The detection unit includes a humidity sensor for producing a humidity detection signal by detecting a humidity, a temperature sensor for producing a temperature detection signal by detecting a temperature surrounding the humidity sensor, and a common power supply line connected to the humidity sensor and the temperature sensor. The controller includes an application circuit for applying a power supply voltage to the humidity sensor and the temperature sensor through the common power supply line, a read circuit for producing a read-out humidity value corresponding to the humidity detection signal and a read-out temperature value corresponding to the temperature detection signal, and a correction circuit for correcting the read-out humidity value based on the read-out temperature value.