1. A signal processor for performing gradation conversion processing using a changed histogram of an image signal, comprising: a feature calculating unit which calculates a feature of the image signal; a local-region extracting unit which extracts a local region, wherein the local region centers on a target pixel, and wherein the target pixel is selected from pixels of the image signal; an interval setting control unit which sets intervals of a pixel value in the changed histogram according to the feature so as to obtain the changed histogram of the local region centering on the target pixel of the image signal, wherein the interval setting control unit sets at least one of: (i) widths of the intervals of the pixel value in the changed histogram, and (ii) a number of the intervals of the pixel value in the changed histogram; a gradation-conversion-characteristic calculating unit which calculates a gradation conversion characteristic with respect to the local region centering on the target pixel, using the changed histogram of the local region centering on the target pixel obtained by using the intervals set at the interval setting control unit; and a gradation converting unit which performs gradation conversion to the local region centering on the target pixel of the image signal, using the gradation conversion characteristic, wherein the feature calculating unit comprises at least one of: an original histogram calculating unit which calculates information on an original histogram of the image signal as the feature; a bit-intensity calculating unit which calculates information on a bit length and an intensity value of the image signal as the feature; and a target subject calculating unit which calculates information on a target subject of the image signal as the feature.