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, wherein the distribution parameters include a minimum value of the pixel signal levels in the one picture, the target value includes a first target value indicating a maximum level of a first region having the minimum value as a starting point on the distribution, in the first step, an average value of the pixel signal levels of the distribution is extracted, in the second step, the first target value within a predetermined range about the average value is set, the distribution parameters include a maximum value of the pixel signal levels in the one picture, and in the second step, a ratio of a value obtained by subtracting the minimum value from the first target value with respect to a value obtained by subtracting the minimum value from the maximum value is found and the ratio is used to determine a second target value.