A cruise-assist image generation device includes a steering-angle information acquisition unit, an additional line generation unit generating a cruise assist additional line corresponding to acquired steering angle information and formed by combination of a given number of segments, and an additional line superimposing unit superimposing the generated cruise assist additional line on a taken image of the vehicle periphery to obtain a composite image. The additional line generation unit generates respective curved lines forming at least some of the segments by quadratic curve approximation using coordinate data corresponding to the acquired steering angle information and including coordinates of points representing each segment, and generating respective straight lines forming any remaining segments by collinear approximation using coordinate data corresponding to the acquired steering angle information.