A self-diagnostic method for PON protection system, in which optical switch-related failure can be previously inspected, and a PON protection system. A circuit is configured while an optical switch is switched so that signal light from an ONU is input to both a normal system OSU and a redundancy system OSU of an optical access network including the ONU, and there are conducted a circuit configuration test in which the signal light to the normal system OSU and the signal light to the redundancy system OSU are monitored for a predetermined period of time, and when the signal light to one of the OSUs and the signal light to the other do not correspond to each other, it is judged as abnormality and an open state test in which an optical switch is opened so that the signal light from the ONU is input to only the normal system OSU of the optical access network including the ONU, a circuit of the redundancy system OSU and the ONU is brought into an open state, the signal light to redundancy system OSU is monitored for a predetermined time of period, and when the signal light to the redundancy system exists, it is judged as abnormality.