To surely detect poor visibility resulting from the fogging of the inside of a window due to moisture, dirt on glass itself, etc.

Brightness is detected at a plurality of observing positions uniformly dispersed over an entire image of a viewing field of a camera 3, and the brightness difference between the observing positions is calculated. The brightness difference is compared with a set value, and if the brightness difference is smaller than the set value, the inside of the glass of window shield 2 is recognized as being fogged, in which case a command to remove fog is output to an air-conditioning control unit 20 to remove fog. Thus poor visibility through the window is surely detected, and the reliability of image recognition is enhanced to exhibit functions fully when, e.g. a camera installed in the interior of a vehicle is used as a running environment sensor in the preventive safety system of the vehicle or used in conjunction with the preventive safety system of the vehicle.