A photoelectric conversion apparatus comprising: a plurality of photoelectric conversion units, each including a plurality of photoelectric conversion elements (1401a, 1401b), the plurality of the photoelectric conversion units including at least one overflow drain region of a first conductivity type, wherein the overflow drain region is constituted by a semiconductor substrate, wherein each of the photoelectric conversion units is configured to add signal carriers generated in the plurality of photoelectric conversion elements (1401a, 1401b) or signals based on the signal carriers, wherein each of the plurality of photoelectric conversion elements (1401a, 1401b) includes a first semiconductor region (203, 204) of the first conductivity type that is configured to collect the signal carriers, wherein the first semiconductor regions (203, 204) included in the photoelectric conversion elements (1401a, 1401b) that are included in each of the photoelectric conversion units are arranged adjacent to each other to sandwich a second semiconductor region of a second conductivity type, the semiconductor substrate is provided in a vertical direction of the first semiconductor regions (203, 204) and the second semiconductor region, characterized in that wherein a height of a potential barrier for the signal carriers generated in a certain region of the second semiconductor region is smaller than a height (309, 605) of a potential barrier for the signal carriers generated in a region between each of the first semiconductor regions (203, 204) and the overflow drain region of the first conductivity type, and wherein the potential barrier of the certain region of the second semiconductor region and the potential barrier of the region are configured such that charge in excess of saturating one of the photoelectric conversion elements (1401a, 1401b) leaks into an adjacent photoelectric conversion element (1401a, 1401b) in the same photoelectric conversion unit before leaking into the overflow drain region. a plurality of photoelectric conversion units, each including a plurality of photoelectric conversion elements (1401a, 1401b), the plurality of the photoelectric conversion units including at least one overflow drain region of a first conductivity type, wherein the overflow drain region is constituted by a semiconductor substrate, wherein each of the photoelectric conversion units is configured to add signal carriers generated in the plurality of photoelectric conversion elements (1401a, 1401b) or signals based on the signal carriers, wherein each of the plurality of photoelectric conversion elements (1401a, 1401b) includes a first semiconductor region (203, 204) of the first conductivity type that is configured to collect the signal carriers, wherein the first semiconductor regions (203, 204) included in the photoelectric conversion elements (1401a, 1401b) that are included in each of the photoelectric conversion units are arranged adjacent to each other to sandwich a second semiconductor region of a second conductivity type, the semiconductor substrate is provided in a vertical direction of the first semiconductor regions (203, 204) and the second semiconductor region, characterized in that wherein a height of a potential barrier for the signal carriers generated in a certain region of the second semiconductor region is smaller than a height (309, 605) of a potential barrier for the signal carriers generated in a region between each of the first semiconductor regions (203, 204) and the overflow drain region of the first conductivity type, and wherein the potential barrier of the certain region of the second semiconductor region and the potential barrier of the region are configured such that charge in excess of saturating one of the photoelectric conversion elements (1401a, 1401b) leaks into an adjacent photoelectric conversion element (1401a, 1401b) in the same photoelectric conversion unit before leaking into the overflow drain region.