It is possible to improve a response of a dynamic range expansion processing. The present image signal processing method includes a first step (ST2) of extracting distribution parameters from a distribution of pixel signal levels composing an image signal of one picture, a second step (ST4, ST5) of determining a target value (BP1, BP2, etc.) of a break point based on extracted distribution parameters for a break point at which an inclination of an image signal level changes with respect to a cumulative change of amount of incident light upon pixels of the imaging portion changes, and a third step (ST6, ST7) of controlling the imaging portion so as to obtain a target value and change an output saturation characteristic of the image signal.