To realize a view obstruction measurement in which an optical transmitter and an optical receiver can be easily, stably and surely interfaced.

An optical transmitter 1 installed in a first spot is provided with a GPS receiver 15, supplies a signal, in which the frequency of a reference clock outputted from the GPS receiver 15 is divided by a frequency divider 16, to a driving circuit 171 as a modulated signal and emits light 3 for measurement modulated by the modulated signal from a light source 172. In an optical receiver 2 installed in a second spot, synchronizing of the received signal converted to an electric signal by an optical receiver 24 is detected by a synchronizing detector 25 while using a synchronizing signal, in which the frequency of a reference cock outputted from a GPS receiver 28 is divided by a frequency divider 27, and that output is recorded in a recorder 26. The GPS receiver 15 is provided in the both of the transmitter 1 and the receiver 2 and both the transmitter and the receiver are synchronized to a universal standard time UTC, so that transmitting and receiving can be synchronized.