A humidity detecting device includes a humidity sensor, which detects humidity using a change in an impedance by humidity, and which generates a humidity detection signal according to application of an AC voltage, a detecting unit that detects the impedance of the humidity sensor based on the humidity detection signal, a switching unit that switches a frequency of the AC voltage to at least a first frequency and a second frequency, the humidity sensor being a first impedance at the first frequency and a second impedance at the second frequency, a temperature sensor that detects ambient temperature of the humidity sensor, a calculating unit that calculates a difference and/or a change rate between the first impedance and the second impedance, a discriminating unit that discriminates the humidity sensor based on the difference and/or the change rate, and a control unit that performs processing corresponding to the discriminated humidity sensor.