A semiconductor photodetector includes a semiconductor substrate including a silicon substrate. The semiconductor substrate includes a second main surface as a light incident surface and a first main surface opposing the second main surface. In the semiconductor substrate, carriers are generated in response to incident light. A plurality of protrusions is formed on the second main surface. The protrusion includes a slope inclined with respect to a thickness direction of the semiconductor substrate. At the protrusion, a (111) surface of the semiconductor substrate is exposed as the slope. The height of the protrusion is equal to or more than 200 nm.