A back-illuminated semiconductor light detecting device includes a light detecting substrate having pixels, and a circuit substrate having signal processing units. For each of the pixels, the light detecting substrate includes avalanche photodiodes respectively having light receiving regions provided in a first main surface side of the semiconductor substrate. In the semiconductor substrate, for each pixel, a trench surrounds at least one region including the light receiving region when viewed from a direction perpendicular to the first main surface. The number of signal processing units is larger than the number of light receiving regions in each pixel, and the number of regions surrounded by the trench in each pixel is equal to or less than the number of light receiving regions in the pixel.