A light detecting device includes a substrate that has a lattice constant. A buffer layer is disposed on the substrate. A gradient layer is formed on the buffer layer opposite to the substrate, and includes a plurality of sublayers that have respectively lattice constants each of which is greater than the lattice constant of the substrate. The sublayers are arranged in a manner that the lattice constants of the sublayers undergo a gradual increase in lattice constant in a direction away from the substrate. A barrier layer is formed on the gradient layer opposite to the buffer layer, and has a lattice constant which is greater than that of the substrate and no smaller than the lattice constants of the sublayers. An absorption layer is formed on the barrier layer opposite to the gradient layer.