A labeling unit that extracts a character included in raster format image data and finds the size of the extracted character, and an approximation method determination unit that determines whether or not to use curve approximation to convert the character based on the found size, are provided. In the case where it has been determined to use curve approximation, the character is converted into a vector format by performing straight-line approximation or curve approximation on each of the various parts of the contour of the character, whereas in the case where it has been determined not to use curve approximation, the character is converted into a vector format by performing straight-line approximation and not performing curve approximation on each of the various parts of the contour of the character.