A photo-detection system includes: a photo-detection apparatus including a light-shielding film, an optically-coupled layer, and a photodetector including first and second photo-detection cells; and an arithmetic circuit that generates, based on first signals and second signals, third signals each representing coherence of light having entered a position of each of the first and second photo-detection cells and generates at least one selected from the group consisting of an average value of the third signals, a standard deviation of the third signals, a ratio between the standard deviation and the average value, and a ratio between an average value of a first portion of the third signals based on light having entered the positions of the first photo-detection cells and an average value of a second portion of the third signals based on light having entered the positions of the second photo-detection cells.