1. An image processing method for changing the gradation levels of a pixel within a 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 of a pixel that is a target of processing, within the 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; judging whether a first difference between the gradation level for the pixel in the background area and the gradation level of the pixel that is the processing target is equal or greater than a first threshold value; and if the first difference is smaller than the first threshold value, changing the value of the pixel that is the processing target to the smaller of two values, a second difference between the gradation level for the pixel in the background area and the first threshold value and the aforementioned minimum value wherein each of the steps is carried out for all pixels within the zone located on the corner.