An on-vehicle fog-presence determining apparatus having image fog-state determining means for determining whether or not an image picked up by an on-vehicle camera mounted on a vehicle is disclosed. The apparatus comprises far-flung brightness acquiring means for acquiring a brightness of a far-flung road region, sky brightness acquiring means for acquiring a brightness of a sky around the vehicle and image fog-state determining means for making judgment using the brightness of the far-flung road region and the brightness of the sky.