Areas, allowed to overlap, are set for respective pixels in an image, and a maximum density value and a minimum density value, among inherent density values of the pixels in each area, are extracted and specified as maximum and minimum density values common to the pixels in the area. Among the density values specified from the overlapping areas, the largest minimum density value is set as a local minimum value and the smallest maximum density value is set as a local maximum value to thereby generate a minimum density plane and a maximum density plane including the pixels having the values. The density values of the pixels at the corresponding positions in the minimum density plane and the maximum density plane are corresponded to the common minimum and maximum values set separately, to thereby linearly convert the whole. Thereby, the pixel density values of the original image are linearly converted simultaneously.