There is provided a solid-state image pickup device comprising a plurality of pixel units, each of which includes a photoelectric conversion element, a signal line which reads out signals from said plurality of pixel units, a first capacitor element which has a first electrode connected to the signal line, an amplifier which has an input terminal connected to a second electrode of the first capacitor element, and a second capacitor element connected between the input terminal and an output terminal of the amplifier, wherein the first capacitor element has a capacitance value which is smaller at the time of execution of an adding mode than at the time of execution of a non-adding mode, and thereby making an amplification factor of the amplifier smaller at the time of execution of the adding mode than at the time of execution of the non-adding mode.