1. An image processing method for changing the gradation levels of a pixel within the rectangular zone of a rectangular image in which a code is embedded, comprising the steps of: obtaining a gradation level of a color that is equivalent to the complementary color of yellow for a pixel in a background area; obtaining a gradation level of the color that is equivalent to the complementary color of yellow for a pixel that is a target of processing, within a zone located on a corner of the rectangular image; obtaining the minimum value of gradation levels of the color that is equivalent to the complementary color of yellow from among pixels included in a predefined section for the pixel that is the processing target; and changing gradation levels so that the difference between the gradation level of the pixel that is the processing target and that of a pixel in the background area is maximized in the surrounding area of the pixel that is the processing target by using the obtained respective gradation levels, wherein each of the steps is carried out for all pixels within the zone located on the corner.