It is possible to allow a user to easily distinguish between an event at a place to be resolved and an event at a place having no problem on a path of a PON communication network to be measured. A light intensity distribution of return light is processed in a time-series order to detect an event at each position on a network. A parameter N1 relating to the total number of splitters present on a path of the network is specified, the number N2 of detections of the total number of splitters detected as an event is recognized, and in a case where “N1>N2”, a last detected event is associated with one optical splitter and is further displayed as an “uncertain splitter” in distinction from a normal splitter.