The present invention aims to provide satisfactory color reproduction without causing color mixing when a color image is formed on a direct heat-sensitive color recording medium. To input values of R, G and B, matrix calculation is performed to intensify chroma. To the output at a matrix calculation unit 10, contrast is decreased with respect to a predetermined color component of a predetermined color, and selective color correction processing is performed to exclude color mixing. Then, colors are converted to Y, M and C respectively. As a result, when an image of M color is formed or when an image of M component is formed during the formation of an image in red color, higher contrast is obtained compared with the case where an image in gray color is formed until the value of M reaches a certain value, and density of said color is made lower than the density during the formation of the image in gray color when the signal value exceeds said value, and color mixing does not occur. The same applies to the case where an image in Y component only is formed and when an image in Y component is formed during the formation of an image in green color.