1. An image processing method comprising the steps of: dividing an inputted image into pixel groups, each of which has a specified size; calculating a pixel group density for each of the divided pixel groups; and calculating an output value of a certain watched pixel based on an absolute density of the watched pixel and a relative density for the watched pixel, the relative density being calculated based on the pixel group density of the pixel group, to which the watched pixel belongs, and the pixel group density of the pixel group adjacent to the pixel group, to which the watched pixel belongs, among the pixel groups in the image.