Converting lightness of each pixel even for images having continuous lightness includes: setting a local area around a pixel for which lightness is converted, in the original image; setting an upper limit conversion function which continuously monotonically increases with respect to the lightness, and determines an output upper limit of lightness conversion; setting a lower limit conversion function which continuously monotonically increases with respect to the lightness, and determines an output lower limit of lightness conversion; calculating upper and lower limit values (upper and lower limit conversion function of lightness of the pixel to be converted); calculating a ratio for setting a value between the upper limit value and the lower limit value according to lightness of each pixel in the local area; and calculating converted lightness of a pixel for which the lightness is converted based on the upper limit value, the lower limit value and the ratio.