An image sensor includes a photoelectric conversion element, a first MOS transistor having having a gate connected to the photoelectric conversion element, a second MOS transistor connected in series with the first transistor, and a third MOS transistor connected in series with the photoelectric conversion element, wherein the threshold voltage of the third MOS transistor is set higher than that of the second MOS transistor. In one embodiment, each of the pixels included in a second group of rows includes a photoelectric conversion element but without the first, second and third MOS transistors. A fourth MOS transistor connects a photoelectric conversion element of the second group to a photoelectric conversion element of a first group, the photoelectric conversion elements of the first group being part of pixels which contain first, second and third MOS transistors.