PURPOSE: To sample a plurality of voltages to be measured within one cycle by recombining the light pulse passing through a variable delay means and the light pulse passing through both of the variable delay means and an auxiliary delay means to allow both of them to be incident to an electro-optical modulator.

CONSTITUTION: A light branch device 207A is provided to the post stage of a variable delay means 202 and an auxiliary delay means 209 is provided on the auxiliary light path 208B branched from said branch device 207A. Further, a discrimination function adding means 203B is provided to the auxiliary light path 208B. This means 203B adds discrimination function to the light pulse passing through the circuit 209 in the same way as the discrimination function adding means 203A provided to a main light path 208A. Both light pulses to which discrimination function added are combined by a light coupler 212 to be sent into an electro-optical modulator 100. The light pulse PM1 passing through the means 209 is delayed by a delay time τ1 from the original light pulse LP and the light pulse branched to the auxiliary light path 208B becomes a light pulse PM2 further delayed by a delay time τ2. As a result, two light pulses PM1, PM2 are obtained from the light pulse coupler 212 within one cycle T.