A photodetecting device includes a semiconductor photodetecting element including a plurality of pixels distributed two-dimensionally and a mount substrate including a plurality of signal processing units arranged to process output signals from the corresponding pixels. The semiconductor photodetecting element includes, for each of the pixels, a plurality of avalanche photodiodes arranged to operate in Geiger mode, a plurality of quenching resistors each electrically connected in series with a respective avalanche photodiodes, and a through-electrode electrically connected to the plurality of quenching resistors. Each of the signal processing units includes a current mirror circuit electrically connected to the plurality of avalanche photodiodes via the corresponding through-electrode and arranged to output a signal corresponding to output signals from the plurality of avalanche photodiodes. The number of signal processing units included on the mount substrate is more than the number of light receiving regions in each of the pixels.