An image sensor, a reading device and a method for setting a resolution are provided for setting the resolution to multiple levels without increasing signal types. The image sensor comprises plural photoelectric converting elements for converting optical signals to electric signals; a group of channel select switches, each channel select switch arranged between a charge output unit of each channel select switch and a signal line corresponding to each photoelectric converting element, wherein the group of the channel select switches can be turned on and off sequentially synchronizing with an externally supplied clock pulse signal; and resolution setting means, wherein when a resolution assignment signal, a resolution assignment timing signal and a resolution assignment period setting signal are input, the resolution setting means sets an on-off control pattern for the group of the channel select switches according to an on-off pattern of the resolution assignment signal at a plurality of timings set by the resolution assignment timing signal in a resolution assignment period set according the resolution assignment setting signal.