A photodetecting device includes a semiconductor substrate including a first principal surface and a second principal surface that oppose each other and a plurality of through-electrodes penetrating through the semiconductor substrate in a thickness direction. The semiconductor substrate includes a plurality of avalanche photodiodes arranged to operate in Geiger mode. The plurality of through-electrodes are electrically connected to the corresponding avalanche photodiodes. The semiconductor substrate includes a first area in which the plurality of avalanche photodiodes are distributed in at least a first direction and a second area in which the plurality of through-electrodes are distributed two-dimensionally. The first area and the second area are distributed in a second direction orthogonal to a first direction when viewed from a direction orthogonal to the first principal surface.