A target line detection device includes a processor configured to execute a process. The process includes: detecting transition points in a brightness image obtained from a brightness component of an input image between pixels with a luminosity gradient in a first direction and pixels with a luminosity gradient in a second direction opposite to the first direction; and, based on a shape or a length or a combination thereof, of lines connecting together transition points that are within a specific distance of each other, extracting a line representing a detection target from the lines connecting together the transition points.