1. An image signal processing method for expanding a dynamic range of an image signal obtained from an imaging portion, said image signal processing method characterized by comprising a first step of extracting distribution parameters from a distribution of pixel signal levels composing the image signal of one picture, a second step of determining a target value of a break point at which an inclination of the change of image signal level with respect to a cumulative change of amount of light incident upon the pixels of the imaging portion changes on the basis of the extracted distribution parameters, and a third step of making an output saturation characteristic of the image signal change, by controlling the imaging portion so that the target value is obtained.