When a monochrome image is converted into a color image, following criteria are applied. a) Black, white, and a color having lightness according to a pixel value are assigned to minimum and maximum pixel values, and a pixel value therebetween, respectively. b) For the pixel value near but excluding the minimum value, a color in a blue color region is assigned. c) For the pixel value near but excluding the maximum value, a color in a yellow color region is assigned. For the pixel value in a range above those near the minimum value but below near the maximum value, a color in a range of hue from the blue color region to the yellow color region by way of a red color region or a color in the range of hue from the blue color region to the yellow color region by way of a green color region is assigned.