A contour pixel is extracted from binarized image data. The contour of the image data is approximated to a straight line based on the extracted contour pixel. It is determined whether a first straight line in the contour obtained by the straight-line approximation includes a portion to be kept straight in the curved-line approximation, based on the relationship between the first straight line and a second straight line connected to the first straight line. In a case where it is determined that the first straight line includes a portion to be kept straight, curved-line approximation is performed by setting a control point on the first straight line at a position closer to an end of the first straight line connected to the second straight line with respect to a midpoint of the first straight line.