Abstract: An optical pulse supplying section supplies an optical pulse to a target optical fiber. A reflection light detecting section outputs an electric signal corresponding to the reflection light from the target optical fiber. Upon reception of the electric signal from this detecting section, a level controlling section controls the level of the received signal according to an initial value or compensation value. An A/D converter receives the output signal of the level controlling section and outputs a digital signal corresponding to the received signal. A parameter input section is for inputting a parameter to determine the level that indicates the reflection light from the target optical fiber. An initial value memory stores the initial value to be set in the level controlling section, in accordance with the parameter from the parameter input section.