An improvement is added to an optical time domain reflectometer for emitting pulsed light of invisible light to a measured optical fiber, receiving return light of the pulsed light by a light detection section, measuring the measured optical fiber, and emitting visible light for visible inspection of a fault point of the measured optical fiber to the measured optical fiber. The optical time domain reflectometer includes an incidence-emission port for emitting the invisible light and the visible light to the measured optical fiber and an output judgment section for judging that a communication light exists in the measured optical fiber based on the light power of the light detection section receiving light incident through the incidence-emission port in a state in which the pulsed light of the invisible light is not emitted.