For calculating an optical sensor's regular-discharge probability, a light detection system includes the optical sensor, an application voltage generating circuit for applying a drive pulse voltage to the optical sensor, a discharge determining portion for detecting the optical sensor's discharge, a first discharge probability calculating portion, a sensitivity parameter storing portion for storing the optical sensor's sensitivity parameters, and a second discharge probability calculating portion for calculating a discharge probability of the optical sensor's regular discharge. The first discharge probability calculating portion calculates a discharge probability in: a first state in which light from an additional light source having a known light quantity is incident on the optical sensor or the additional light source is turned off; and a second state in which the additional light source's turning-on/turning-off status is different from the first state, with the drive pulse voltage's pulse width being the same as the first state.