An optical detector includes: a light-receiving element that is operated in a Geiger mode by application of a service voltage higher than a breakdown voltage and outputs a pulse signal in accordance with reception of light; a voltage setting unit that is capable of selectively applying any one of the service voltage and a test voltage lower than the breakdown voltage to the light-receiving element; and an abnormality determination unit that performs an abnormality determination process of determining that the light-receiving element is abnormal when the number of pulse signals output from the light-receiving element operated under the test voltage is equal to or greater than a predetermined threshold, and determining that the light-receiving element is normal when the number of pulse signals is smaller than the threshold.