A window fog detecting apparatus has an optical type fog detecting sensor for optically detecting fog occurring at a window glass, a humidity sensor for detecting a relative humidity of air at an interior side of the window glass, and a sensor output correcting unit. When a fog occurrence at the window glass is determined based on the output values of the optical type fog detecting sensor, the sensor output correcting unit corrects the relative humidity detected by the humidity sensor based on output values of the optical type fog detecting sensor.