A driver's gaze distribution is detected. Based on the detected distribution, an image corresponding to a driver's visual-field is extracted from images photographed by a front monitor camera and is accumulated. A first driver's gaze probability distribution necessary for steering operation is computed based on the accumulated images, and a second driver's gaze probability distribution that is expected is computed based on visual characteristics within the images. An ideal gaze probability distribution is obtained by adding up the first and the second probability distributions. Presence of a risk is determined when the difference between the driver's gaze and the ideal gaze probability distribution becomes a given threshold. A windshield display or a speaker outputs the determined result.