A semiconductor light detecting element comprising: a silicon substrate having a semiconductor layer having a first impurity concentration, and an epitaxial semiconductor layer grown on the semiconductor layer and having a second impurity concentration lower than the first impurity concentration; and a conductor provided on a front surface of the epitaxial semiconductor layer, the front surface facing away from the semiconductor layer, wherein a photosensitive region is formed in the epitaxial semiconductor layer, wherein an asperity is formed at least in a back surface of the semiconductor layer, the back surface facing away from the photosensitive region, and wherein the asperity is optically exposed and configured to scatter, diffuse or reflect light incident from a side of the back surface into the silicon substrate, wherein the conductor comprises a photogate electrode provided on the front surface of the epitaxial semiconductor layer, and first and second gate electrodes (TX1, TX2) provided adjacent to the photogate electrode on the front surface of the epitaxial semiconductor layer, the semiconductor light detecting element further comprising: first and second semiconductor regions (FD1, FD2) formed in the epitaxial semiconductor layer, for reading out respective charges flowing from a region immediately below the photogate electrode to immediately below the first and second gate electrodes (TX1, TX2), wherein the asperity is formed at least in a part of the back surface opposed to the region immediately below the photogate electrode in the semiconductor layer, characterized in that the asperity is irregular. a silicon substrate having a semiconductor layer having a first impurity concentration, and an epitaxial semiconductor layer grown on the semiconductor layer and having a second impurity concentration lower than the first impurity concentration; and a conductor provided on a front surface of the epitaxial semiconductor layer, the front surface facing away from the semiconductor layer, wherein a photosensitive region is formed in the epitaxial semiconductor layer, wherein an asperity is formed at least in a back surface of the semiconductor layer, the back surface facing away from the photosensitive region, and wherein the asperity is optically exposed and configured to scatter, diffuse or reflect light incident from a side of the back surface into the silicon substrate, wherein the conductor comprises a photogate electrode provided on the front surface of the epitaxial semiconductor layer, and first and second gate electrodes (TX1, TX2) provided adjacent to the photogate electrode on the front surface of the epitaxial semiconductor layer, the semiconductor light detecting element further comprising: first and second semiconductor regions (FD1, FD2) formed in the epitaxial semiconductor layer, for reading out respective charges flowing from a region immediately below the photogate electrode to immediately below the first and second gate electrodes (TX1, TX2), wherein the asperity is formed at least in a part of the back surface opposed to the region immediately below the photogate electrode in the semiconductor layer, characterized in that the asperity is irregular. first and second semiconductor regions (FD1, FD2) formed in the epitaxial semiconductor layer, for reading out respective charges flowing from a region immediately below the photogate electrode to immediately below the first and second gate electrodes (TX1, TX2), wherein the asperity is formed at least in a part of the back surface opposed to the region immediately below the photogate electrode in the semiconductor layer, characterized in that the asperity is irregular.