A travel road detector includes: an image pickup section producing a road image of a forward travel road in front of a driver's vehicle; a setting section setting small areas for lane marker detection; a lane marker detector detecting lane markers on both sides of a travel lane for the driver's vehicle; a first calculator calculating a travel road model parameter representing a shape of the forward travel road when a both-side lane marker detecting state is satisfied; a forward vehicle detector detecting a height of a predetermined part of a forward vehicle traveling ahead of the driver's vehicle when the both-side lane marker detecting state is present; and a second calculator calculating the travel road model parameter from information on the lane marker detection points, and information on the height of the predetermined part of the forward vehicle detected with the forward vehicle detector.