At the time of real-time measurement, when a measurement position is specified by a marker, a marker level acquiring section acquires a waveform level from waveform data stored in a waveform memory. A level comparing section determines whether the waveform level acquired by the marker level acquiring section falls within an effective measurement level range preset correspondingly with attenuator (ATT) values. When the level comparing section determines that the waveform level does not fall within the effective measurement level range, an ATT value change section changes the ATT values into new ATT values at which a waveform level falls within the corresponding effective measurement level range. A control section allows a measured optical fiber to be measured based on the new ATT value, so as to enable waveform measurement with a good S/N ratio of not less than a predetermined value based on a measurement waveform displayed on a screen of a display section.