A light detector is provided to include a detection module, and a monitoring SPAD. The detection module includes an SPAD that is an avalanche photodiode operable in Geiger mode; the detection module is configured to perform light detection by applying, to the SPAD, a reverse bias voltage in a reverse direction. The monitoring SPAD has characteristics identical to characteristics of the SPAD in the detection module. The reverse bias voltage is generated to be applied to the SPAD in the detection module based on (i) a reference breakdown voltage set from a breakdown voltage generated across the monitoring SPAD in Geiger mode and (ii) a predetermined excess voltage.