1. An image signal processing device comprising: a feedback clamp circuit that increases or decreases a clamp parameter so that a black level approaches a target value while controlling a change amount of the clamp parameter, which sets the black level, based on the black level read out from OB pixels; an OB clamp circuit that clamps an image signal read out from effective pixels and calculates a clamp parameter correction coefficient that controls the change amount of the clamp parameter based on the black level read out from the OB pixels; a gain adjustment circuit that adjusts white balance and gain of an image signal output from the clamp circuit; a color separation circuit that converts an image signal output from the gain adjustment circuit into a color separation signal; and an automatic level control circuit that adjusts a digital gain and an analog gain based on a luminance signal extracted in the color separation circuit, and outputs the digital gain to the gain adjustment circuit and outputs the analog gain to the feedback clamp circuit.