PURPOSE: To convert full-color image data, represented with the three primary colors, into image data of two primary colors without causing image deteriora tion such as a contrast decrease.

CONSTITUTION: This method has a process for finding luminance information from respective pieces of information of R, G, and B, a process for finding a point on a luminance axis from the value of the luminance information on an RG space wherein the values of the R information and G information are set on mutually orthogonal R and G axes and the luminance axis extended at 45° to those R and G axes is set, a process for finding an R-G axis segment which passes the point on the luminance axis and crosses the luminance axis at right angles, a process for finding difference between the values R and G, a process for finding a point moved from the mid-point of the R-G axis segment toward the R axis by the difference value when the value R is larger than the value G or a point moved from the mid-point of the R-G axis segment toward the G axis side by the difference value when the value G is larger than the value R, and a process for finding the R coordinate and G coordinate of the point moved on the R-G axis segment as conversion results.