Abstract: A processing unit notifies a drive unit of a designated number N (=1, 2, . . . ) The drive unit supplies a drive signal of a frequency f.sub.0 to a target measurement device, and supplies a modulation signal of a frequency N.multidot.f.sub.0 +.DELTA.f to a light source. In this state, an intensity-modulated light beam is emitted from a light source, incident on an E-O probe through an optical system including a PBS and a wave plate, influenced by a periodical voltage waveform generated at a target measurement portion at a fundamental period 1/f.sub.0 to modulate the polarized state, and output from the E-O probe. The optical signal whose polarized state is modulated is input to a photodetector after a polarization direction is selected by the PBS, and heterodyne-detected. The processing unit reproduces the waveform of the voltage signal generated at the target measurement portion by Fourier transform on the basis of the obtained calculation of photodetection signal and displays the waveform.