A solid-state image-pickup device including a multiplier configured to perform digital-gain processing for a sensor signal output from an analog-to-digital converter configured to convert an analog signal of an image sensor into a digital signal is provided. The solid-state image-pickup device includes a signal generator configured to generate randomly distributed signals, and an adder configured to add the randomly distributed signals to at least one signal with step-like discrete gradation, the signal being output from the multiplier.