There are provided a labeling portion that extracts a character included in raster format image data, a complexity calculation portion that obtains a degree of complexity indicating complexity of the character, an approximation method determination portion that determines whether or not to use curve approximation to convert the character based on the degree of complexity thus obtained. In the case where it has been determined to use the curve approximation, the character is converted into a vector format by performing straight-line approximation or curve approximation on each part of a contour of the character, whereas in the case where it has been determined not to use the curve approximation, the character is converted into a vector format by performing the straight-line approximation on each part of the contour of the character without performing the curve approximation.