A solid-state imaging device is provided in which sensitivity is prevented from lowering even when signals of pixels are mixed. The solid-state imaging device includes a plurality of pixel units each of which has a photoelectric conversion element, and is capable of summing signals corresponding to respective outputs of the photoelectric conversion elements of the pixel units. The device includes: a plurality of capacitors, each of which individually accumulates electric charges corresponding to a signal outputted from the associated photoelectric conversion element; and a plurality of MOS transistors which are alternately connected with the associated capacitor. By disconnecting the MOS transistor, the electric charges of the signal outputted from each of the photoelectric conversion elements are accumulated in each associated capacitor, and by conducting the MOS transistors to sum the signals of the pixel units, the capacitors are connected in series.