A photodetector includes: a semiconductor substrate having a first main surface and a second main surface; a first semiconductor layer that is of a first conductivity type, and is included in the semiconductor substrate and closer to the first main surface than to the second main surface; a second semiconductor layer that is of a second conductivity type different from the first conductivity type, and is included in the semiconductor substrate and interposed between the first semiconductor layer and the second main surface; a multiplication region that causes avalanche multiplication to a charge generated in the semiconductor substrate through photoelectric conversion; a circuit region disposed alongside the first semiconductor layer in a direction parallel to the first main surface; at least one isolation transistor disposed in the circuit region; and an isolation region interposed between the first semiconductor layer and the circuit region.