An image pickup apparatus comprising: a plurality of incident ray conversion means each configured to convert incident rays into charge; a plurality of analog-to-digital conversion means each provided with a comparator and configured to convert a signal output from the incident ray conversion means into a digital signal; a plurality of counters each configured to output a count signal obtained by counting clock signals; reference signal output means configured to output a reference signal, of which a signal level is changed dependently on time, to each of the comparators of the plurality of analog-to-digital conversion means; and control means configured to output start signals for starting clock signal counting to each of the plurality of counters, wherein first analog-to-digital conversion means among the plurality of analog-to-digital conversion means is configured to generate the digital signal based on the count signal output by a first counter among the plurality of counters and a result of a comparison conducted by the comparator between the signal output by the incident ray conversion means and the reference signal, wherein second analog-to-digital conversion means among the plurality of analog-to-digital conversion means is configured to generate the digital signal based on the count signal output by a second counter among the plurality of counters and a result of a comparison conducted by the comparator between the signal output by the incident ray conversion means and the reference signal, wherein the image pickup apparatus is configured to obtain a delay amount of the reference signal output to the comparator of the second analog-to-digital conversion means from the reference signal output means with respect to the reference signal output to the comparator of the first analog-to-digital conversion means from the reference signal output means, and wherein the control means is configured to output the start signals such that a start of counting the clock signals by the second counter is delayed by a delay corresponding to the obtained delay amount in relation to a start of counting the clock signals by the first counter. a plurality of incident ray conversion means each configured to convert incident rays into charge; a plurality of analog-to-digital conversion means each provided with a comparator and configured to convert a signal output from the incident ray conversion means into a digital signal; a plurality of counters each configured to output a count signal obtained by counting clock signals; reference signal output means configured to output a reference signal, of which a signal level is changed dependently on time, to each of the comparators of the plurality of analog-to-digital conversion means; and control means configured to output start signals for starting clock signal counting to each of the plurality of counters, wherein first analog-to-digital conversion means among the plurality of analog-to-digital conversion means is configured to generate the digital signal based on the count signal output by a first counter among the plurality of counters and a result of a comparison conducted by the comparator between the signal output by the incident ray conversion means and the reference signal, wherein second analog-to-digital conversion means among the plurality of analog-to-digital conversion means is configured to generate the digital signal based on the count signal output by a second counter among the plurality of counters and a result of a comparison conducted by the comparator between the signal output by the incident ray conversion means and the reference signal, wherein the image pickup apparatus is configured to obtain a delay amount of the reference signal output to the comparator of the second analog-to-digital conversion means from the reference signal output means with respect to the reference signal output to the comparator of the first analog-to-digital conversion means from the reference signal output means, and wherein the control means is configured to output the start signals such that a start of counting the clock signals by the second counter is delayed by a delay corresponding to the obtained delay amount in relation to a start of counting the clock signals by the first counter.