Provided is, for example, a photodiode in which extension of the sensitivity range to a longer wavelength in the near-infrared region can be achieved without increasing the dark current. A photodiode according to the present invention includes an absorption layer 3 that is positioned on an InP substrate 1 and has a type-II multiple-quantum well structure in which an InGaAs layer 3a and a GaAsSb layer 3b are alternately layered, wherein the InGaAs layer or the GaAsSb layer has a composition gradient in the thickness direction in which the bandgap energy of the InGaAs or the GaAsSb decreases toward the top surface or the bottom surface of the layer.