A hetrajunction pin photodiode having a structure capable of high frequency response and saturation output current. The pin photodiode is formed by: a first semiconductor layer (11) of a first conduction type; a second semiconductor layer (12) of a second conduction type; a third semiconductor layer (13) sandwiched between the first and second semiconductor layers, having a doping concentration lower than those of the first and second semiconductor layers; a fourth semiconductor layer (14) of the first conduction type, provided at one side of the first semiconductor layer opposite to a side at which the third semiconductor layer is provided; and a cathode electrode (16) and an anode electrode (15) connected directly or indirectly to the second semiconductor layer (12) and the fourth semiconductor layer (14), respectively. The first semiconductor layer (11) has a bandgap energy by which a charge neutrality condition is maintained in at least a part of the first semiconductor layer and the first semiconductor layer is made to function as a light absorption layer, while the second and third semiconductor layers (12,13) have bandgap energies by which the second and third semiconductor layers are made not to function as a light absorption layer, and the fourth semiconductor layer (14) has a bandgap energy greater than that of the first semiconductor layer.