A window fog detecting system for detecting fog on a windshield of a vehicle, including: a glass temperature detecting system including a contact temperature sensor attached to a portion of the windshield on the driver's seat side of the center in a width direction of the windshield, or to a portion where ceramic print is applied on the upper-side periphery of the windshield; a humidity detection device which detects the humidity of air in a vicinity of the inner surface of the windshield; and a judging device which judges the presence of fog on the windshield, on the basis of the detected values of the glass temperature detecting system and the detected values of the humidity detection device.