A humidity detecting apparatus includes a humidity sensor for detecting a relative humidity of air on an inner side of a window glass, an air temperature sensor for detecting a temperature of the air, a glass temperature sensor for detecting a temperature of the window glass, and a glass surface relative humidity calculation unit for calculating a glass surface relative humidity based on output values of the humidity sensor, the air temperature sensor and the glass temperature sensor. A heat conductive member is disposed between the glass temperature sensor and the window glass.