To provide a method for specifying an obstacle ONU (Optical Network Unit) which outputs an interruption light.

When detecting the occurrence of an obstacle ONU, a CPU (24) switches a switch 32 so that an up-light electric signal is supplied to a power meter 38. The CPU (24) measures an interruption light power (P0) by the power meter (38). The CPU (24) designates each ONU (18-1-n) in order with a discovery procedure, makes the ONU establish a logical link, makes the ONU output the up-light signal using a whole up bandwidth, and makes the power meter (38) measure an up-light power (Pi) at that time. When the up-light power (Pi) is equal to or less than the result of the interruption light power (P0) to which a predetermined threshold is added, a subscriber optical terminal device is made an obstacle ONU candidate. Within the obstacle ONU candidates, a subscriber optical terminal device optically closest to a station side optical terminal device is specified as an obstacle ONU.

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