Abstract

A light branching section applies a light pulse emitted in synchronism with a drive pulse (DP), to an optical fiber line path, and receives the light returning from the line path. An A/D converter performs equivalent sampling on the optical signal contained in the returning light, in synchronism with the sampling pulse (SP) of period T0. A measuring control circuit (MCC) generates DP a number (M) of times. The circuit generates N number of SPs every time it outputs DP. It outputs the SPs, at a time delayed by ƊT that corresponds to 1/M of the period T0. A clock signal generation circuit outputs to the MCC a first clock signal of a period T1 and a second clock signal of a period T2 different by Ɗt from the period T1. The MCC imparts a delay ƊT to the SPs, on the basis of the Ɗt.