1. A signal processor for performing gradation conversion processing using a 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 centering a target pixel of the image signal; an interval setting control unit which sets intervals of a pixel value in the histogram according to the calculated feature so as to obtain the histogram of the local region of the image signal; a gradation-conversion-characteristic calculating unit which calculates a gradation conversion characteristic with respect to the local region, using the histogram of the local region 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 of the image signal, using the gradation conversion characteristic.