1. An image processing method of generating conversion data, which is used in conversion between data on a device-dependent color space and data on a device-independent color space, from a calorimetric value, comprising steps of: acquiring media white and black points; calculating a gray-balance black points from the media white and black points; converting the calorimetric value using the media white point and the gray-balance black point; and calculating the conversion data using the converted calorimetric value to perform gray-balance correction, wherein the gray-balance black point is calculated by changing lightness of the media white point to lightness of the media black point.