An image sensor having a plurality of pixels two-dimensionally arranged in a row direction and a column direction, comprises a plurality of pixel amplifiers which are arranged in the row direction for each column, and one of which is shared by at least two pixels of each column; two output lines arranged for each column, to either of which signals are output from the plurality of pixel amplifiers arranged in the row direction for each column; and an averaging unit which averages signals output from the two output lines arranged for the same column.