PURPOSE: To eliminate unneeded reflected pulses from a light branch method as well as a loss of insertion by use of a light branch unit in which the 1st and 2nd ports conduct in the 1st state and the 2nd and 3rd ports conduct in the 2nd state.

CONSTITUTION: A light pulse sent out of light emission source 2 enters light branch unit 11 by way of the 1st port 10 and, after being deflected by electric or acousto- optical element 15, enters optical fiber 4 connected to the 2nd port 12. At this time, deflection signal supply source 13, only until the light pulse sent out of light emission source 2 passes through light branch unit 11, supplies a deflection signal to acousto-optical element 15 and then turns off. Part of the light pulse sent to optical fiber 4 is reflected at fault point 3 and then enters light branch unit 11 from the 2nd port 12 by way of optical fiber 4 again and it advances straight in light branch unit 1 and reaches photodetector 6 by way of the 3rd port 14.