A semiconductor stacked body includes: a first semiconductor layer containing a group III-V compound semiconductor and being a layer whose conductivity type is a first conductivity type; a quantum-well light-receiving layer containing a group III-V compound semiconductor; a second semiconductor layer containing a group III-V compound semiconductor; and a third semiconductor layer containing a group III-V compound semiconductor and being a layer whose conductivity type is a second conductivity type. The first semiconductor layer, the quantum-well light-receiving layer, the second semiconductor layer, and the third semiconductor layer are stacked in this order. The concentration of an impurity that generates a carrier of the second conductivity type is 1×1014 cm−3 or more and 1×1017 cm−3 or less in the second semiconductor layer.