There is provided a method for evaluating the contamination by quantifying the surface contamination of an object such as a window glass plate, a wall, or the like in such a manner that the quantified value is close to the visual sensation of human being. The method comprises the steps of taking a color image of the surface of the object, converting a color image data denoting the color image into a monochromatic image data, dividing a monochromatic image denoted by the monochromatic image data into a dot matrix to obtain lightness per dot, selecting an area to be evaluated on the dot matrix, calculating a standard deviation of lightness from the distribution of lightness of the selected area.