An image sensor including a plurality of photoelectric converter elements each operable to convert an optical signal into an electric signal, a plurality of channel selector switches selectively turned on and off to selectively connect and disconnect output portions of the corresponding photoelectric converter elements to and from a common signal line, in synchronization with a clock pulse signal, and a resolution setting portion operable to receive a resolution setting timing signal, and first and second resolution setting signals, and to select one of a plurality of on-off control patterns of the channel selector switches, for thereby setting an image resolution value of the image sensor, on the basis of on-off states of the first and second resolution setting signals upon at least one of rising and falling of the resolution setting timing signal. Also disclosed are an image reading device including the image sensor, and an image resolution setting method using the resolution setting timing signal and the first and second resolution setting signals.