A semiconductor photodetector has at least one unit pixel having a photoelectric conversion part, a charge storage part, and a detection circuit. The photoelectric conversion part includes a charge multiplication region in which incident light is converted into a charge, and the charge is multiplied by avalanche multiplication. The charge storage part is connected to the photoelectric conversion part and stores a signal charge from the photoelectric conversion part. The detection circuit is connected to the charge storage part, converts the signal charge stored in the charge storage part into a voltage, passes the voltage through an amplifier to amplify the voltage, and outputs the amplified voltage.