When generating outline data, the contour pixels of the binarized image data are first extracted. Based on the extracted contour pixels, the contour of the image data is approximated to a straight line. In the straight-line approximation process, the distance between adjacent contour pixels among the extracted contour pixels is calculated. It is determined based on the result of the comparison between the length of the first straight line and the length of the second straight line whether the first straight line is used as a contour of the image data. The first straight line connects the first contour pixel and the second contour pixel adjacent to the first contour pixel. The second straight line connects the second contour pixel and the third contour pixel adjacent to the second contour pixel.