1. An outlining method comprising the steps of: extracting contour pixels from binarized image data; and performing straight-line approximation for a contour of said image data based on said extracted contour pixels, said step of performing straight-line approximation including the steps of, calculating a distance between adjacent contour pixels among said extracted contour pixels, and determining based on a result of a comparison between a length of a first straight line connecting a first contour pixel and a second contour pixel adjacent to said first contour pixel and a length of a second straight line connecting said second contour pixel and a third contour pixel adjacent to said second contour pixel whether said first straight line is used as a contour of said image data.