An image sensor comprises plural sets of a unit pixel outputting a pixel signal based on an electric charge generated through photoelectric conversion and a conversion unit converting the pixel signal into a digital signal. A reference signal source generates reference signals and supplies the generated reference signals to the conversion unit through signal lines. The conversion unit of each set comprises a comparator which compares the level of the reference signal with that of the pixel signal, a count circuit which counts a clock based on the comparison processing, a selection circuit which selects, among the signal lines, a signal line to be selectively connected to the input of the comparator, and a switch which selectively connects the selected signal line to the input of the comparator, and selectively connects a load to an unselected one of the signal lines.