PURPOSE: To suppress a detection error and to improve a humidity detecting accuracy by calculating humidity of the air to be humidified in a state that a variation in a saturation efficiency is accurately grasped.

CONSTITUTION: Humidity detecting means 27, 28 respectively detect temperature th of the air Ah to be humidified and temperature tm of humidified air Am passed through a moisture layer 21. Memory means 25a stores an absolute humidity difference of the air Ah to be humidified and the air Am, mutual change characteristics with a saturation efficiency in the layer 21 and humidity change gradient characteristics of the layer 21. Calculating means 25b obtains the difference of the air Ah and the air Am based on the gradient characteristics, detected temperature th of the air to be humidified and the temperature tm, and calculates relative humidity or absolute humidity of the air Ah based on the difference, the change characteristics and the temperature th. Thus, a detection error can be suppressed, and a humidity detecting accuracy is improved.