There is disclosed a photodetector which is capable of attaining an enhanced detection accuracy, and at the same time permits reduction in the size and manufacturing costs thereof. An avalanche photodiode detects an incident light, in a state of a predetermined bias voltage set thereto. A cooler cools the avalanche photodiode to a predetermined cooling temperature. An amount of an incident signal light incident on the avalanche photodiode is detected based on a detection signal from the avalanche photodiode. A control block adjusts at least one of the bias voltage and the predetermined cooling temperature, thereby holding a value of the detection signal from the avalanche photodiode generated in a state of the incident light being blocked from impinging on the avalanche photodiode, within a predetermined tolerance range.