An image sensor in which a first chip and a second chip are stacked comprises: a pixel unit; output lines each configured to output a pixel signal from the pixel unit; and an analog-digital converter provided for each of the output lines. The analog-digital converter comprises a plurality of sets of the following configuration: reference signal generation units configured to generate a reference signal, a comparison unit configured to compare a signal level of a pixel signal output to one of the output lines with a signal level of the reference signal, a counter configured to count until the signal level of the pixel signal coincides with the signal level of the reference signal compared by the comparison unit. The pixel unit is arranged in the first chip and the analog-digital converter is arranged in the second chip.