1. An image processing method for adjusting a luminance value of each pixel included in an input image, the image processing method comprising the steps of: (S 4 ) demarcating a closed region β(x, y) made up of a plurality of pixels in the input image as a closed target region, repeatedly moving the closed target region within the input image by a predetermined number of pixel units to respectively calculate a maximum value (e max ) and a minimum value (e min ) of luminance energy defined as a luminance arrangement in the closed target region based on the maximal luminance energy value and the minimal luminance energy value of each and every one of the closed target region and its moved versions, and calculating a difference data (e diff ) of the luminance energy each time the closed target region is moved, where e diff =e max −e min ; (S 5 ) applying a linear transform, based on a linear function, to a luminance value of each coordinate based on the difference data e diff , wherein applying a linear transform includes steps (S 5 - 1 )-(S 5 - 4 ) as follows: (S 5 - 1 ) for an arbitrarily defined pixel, a width of 0 to e min and a width of e max to I 2 are compared, whereby a value of whichever width is smaller is denoted by L such that L=min(I 2 −e max , e min ); (S 5 - 2 ) if e′ max =e max +L, e′ min =e min −L, and e′ diff =e′ max −e′ min , a luminance value P(i,j) on a shifted pixel is linearly transformed such that e min to e max are transformed to e′ min to e′ max , and the transformed value is denoted by p′(i, j); (S 5 - 3 ) applying steps (S 5 - 1 ) and (S 5 - 2 ) to all the pixels; and (S 5 - 4 ) obtaining a maximum value and a minimum value of the transformed values and performing a, linear transform such that the maximum value and the minimum value fall within a range in accordance with Expression A, 
 p′ (i,j) =e′ diff ×( p (i,j) −e min )/ e diff (Expression A).