A traveling lane detector according to the present invention includes: an imaging unit mounted on a vehicle to take a road surface image; and an image processor performing image processing on the image to detect lane marks on the road surface. The image processor judges whether the vehicle is crossing the lane marks, and, when the vehicle is not crossing any of the lane marks, defines first and second windows, in the image, for detecting the lane marks located respectively on left and right parts of a road surface in front of or behind the vehicle, performs image processing on the image in each of the windows to detect the lane marks, and, when the vehicle is crossing any of the lane marks, defines a third window including the currently-crossed lane mark in the image, performs image processing on the image in the third window to detect the lane mark.