1. A tone transformation apparatus transforming a tone of an original image, the apparatus comprising: a signal obtaining unit which extracts or generates a first signal Z[i, j] that reflects brightness, or brightness and color difference of a pixel [i, j] of the original image; a neighborhood processing unit which extracts or generates a second signal ZL[i, j] that reflects brightness and color difference of a neighborhood area of the pixel [i, j]; a gain generator which generates an added composite signal Z 1 =(β·Z +α·ZL) of the first signal Z[i, j] and the second signal ZL[i, j], and determines a transformation gain k for the pixel [i, j] by substituting the added composite signal Z 1 for Z 1 in a function k=k (Z 1 ) of a predetermined gain curve; and a tone transformation unit which performs tone transformation by multiplying, by the transformation gain k for said pixel [i, j], a color component of said pixel [i, j] or a signal component generated from the color component.