1. A line segment detector including a first line-segment detecting part comprising: a vertical line detecting part that detects whether or not an input image has a vertical line; a horizontal line detecting part that detects whether or not the input image has a horizontal line; a left-to-right downward slanting line detecting part that detects whether or not the input image has a left-to-right downward slanting line; a left-to-right upward slanting line detecting part that detects whether or not the input image has a left-to-right upward slanting line; and a line-segment detecting/synthesizing section that calculates OR operation of output values from the vertical line detecting part, the horizontal line detecting part, the left-to-right downward slanting line detecting part, and the left-to-right upward slanting line detecting part, the line segment detector further comprising: a re-determining unit that re-determines whether or not the detection result output from the line-segment detecting/synthesizing section is a line segment.