PURPOSE: To highly precisely measure an electric signal to be measured of an arbitrary repeating frequency.

CONSTITUTION: A drive signal transmitted to a circuit 2 to be measured is branched and given to a divider 3. The divided 3 drives the repeating frequency fsig of the drive signal into a natural number (n) and a divided signal of the repeating frequency fL (fL=fsig/n) is given to a phase shifter 4. The phase shifter 4 receives a timing control voltage signal (saw-tooth-wave) from a signal generator 5 and the phase of the divided signal is changed by a constant quantity. Thereby, generation timing of laser pulse light is changed by a constant quantity and the waveform of an electric signal to be measured is sampled repeatedly. Addition average processing of the waveform value of the sampled electric signal to be measured is performed by an averaging device 10.