A reference pixel PB is specified for a specified pixel P0 whose (luminance) value is below a threshold, for example, in the RGB data obtained in image pickup. The reference pixel PB should be on a straight line passing through the specified pixel P0 and the center O of the image and spaced from the specified pixel P0 by a distance, dr, depending on a reduction or expansion rate of a blue component (or B) image of the optical image corresponding to an optical zoom magnification in the image pickup. Only when the B value of the specified pixel P0 is greater than that of the reference pixel PB, the B value of the specified pixel P0 is replaced with that of the reference pixel PB. By expanding or reducing the blue component image alone for the original RGB data, image data is obtained in which the magnification chromatism is corrected. Thus, a chromatism correction apparatus that corrects a chromatism in the picked-up image without using a special lens in the optical system, and a program for use in implementing such apparatus are provided.