Driving support techniques using in-vehicle sensors have attracted much at- tentions and have been applied to practical systems. We focus on supporting drivers in poor visibility conditions. Fog is one of the causes that lead to lack of visibility. In this paper, we propose a method of judging fog density by using in-vehicle camera images and millimeter-wave (mm-W) radar data. This method determines fog density by evaluating both the visibility of a preceding vehicle and the distance to it. Experiments showed that judgments made by the proposed method achieve a precision rate of 85% when compared to the ground-truth obtained by human judgments.