1. A curve rendering device for rendering a curve, having a width and defined by a center line and a line width, by linear approximation of an outline of the curve, the curve rendering device comprising: a non-transitory memory storing a program; and a hardware processor that executes the program causing the curve rendering device to operates as: a segmentation point calculating unit configured to calculate a plurality of segmentation points on the center line for segmenting the center line; an offset point calculating unit configured to calculate, for each of the segmentation points, two offset points separated from the segmentation point by an equal distance on a normal to the center line at the segmentation point; a distance calculating unit configured to calculate a distance between offset points in each of two pairs selected from four offset points calculated for two adjacent segmentation points, the offset points in each pair being located on a same side with respect to a straight line connecting the two adjacent segmentation points; a judging unit configured to calculate a ratio between the calculated two distances using a shorter one of the distances as a denominator, and judge whether or not the ratio is greater than a predetermined value; and a curve forming unit configured to approximate, between two partial outlines which are portions of the outline delimited at the respective normals at the two adjacent segmentation points, the longer one of the two partial outlines by using a greater number of line segments than line segments used to approximate the shorter one of the two partial outlines when the ratio is judged to be greater than the predetermined value.