A photodetecting device comprising: a semiconductor photodetecting element that includes a semiconductor substrate having a first principal surface and a second principal surface opposing each other, and a plurality of pixels distributed two-dimensionally in the semiconductor substrate; and a mount substrate on which the semiconductor photodetecting element is mounted, the mount substrate including a plurality of signal processing units; arranged to process output signals from the respective pixels, wherein, for each of the pixels, the semiconductor photodetecting element includes: a plurality of avalanche photodiodes each including a light receiving region provided in the first principal surface side of the semiconductor substrate, and arranged to operate in Geiger mode; a plurality of quenching resistors disposed on the first principal surface side of the semiconductor substrate, and each electrically connected in series with a respective avalanche photodiodes; and a through-electrode electrically connected to the plurality of quenching resistors, and penetrating through the semiconductor substrate in a thickness direction, wherein the light receiving regions of the plurality of avalanche photodiodes are two-dimensionally distributed for each of the pixels, each of the signal processing units includes a current mirror circuit (34,44) 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, and the number of the plurality of signal processing units included on the mount substrate is more than the number of light receiving regions in each of the pixels. a semiconductor photodetecting element that includes a semiconductor substrate having a first principal surface and a second principal surface opposing each other, and a plurality of pixels distributed two-dimensionally in the semiconductor substrate; and a mount substrate on which the semiconductor photodetecting element is mounted, the mount substrate including a plurality of signal processing units; arranged to process output signals from the respective pixels, wherein, for each of the pixels, the semiconductor photodetecting element includes: a plurality of avalanche photodiodes each including a light receiving region provided in the first principal surface side of the semiconductor substrate, and arranged to operate in Geiger mode; a plurality of quenching resistors disposed on the first principal surface side of the semiconductor substrate, and each electrically connected in series with a respective avalanche photodiodes; and a through-electrode electrically connected to the plurality of quenching resistors, and penetrating through the semiconductor substrate in a thickness direction, a plurality of avalanche photodiodes each including a light receiving region provided in the first principal surface side of the semiconductor substrate, and arranged to operate in Geiger mode; a plurality of quenching resistors disposed on the first principal surface side of the semiconductor substrate, and each electrically connected in series with a respective avalanche photodiodes; and a through-electrode electrically connected to the plurality of quenching resistors, and penetrating through the semiconductor substrate in a thickness direction, wherein the light receiving regions of the plurality of avalanche photodiodes are two-dimensionally distributed for each of the pixels, each of the signal processing units includes a current mirror circuit (34,44) 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, and the number of the plurality of signal processing units included on the mount substrate is more than the number of light receiving regions in each of the pixels.