An electric field waveform of an optical signal is precisely measured with high time resolution. Particularly, determination of inter-symbol interference has been difficult. Output light from the laser source is divided into first and second portions. The first portion is modulated by an optical modulator. The second portion is delayed by a delay line for the same quantity of delay as that of the first portion. The first and second portions are fed to a phase diversity circuit to configure a homodyne interferometer. An optical input sampling oscilloscope stabilizes a variable optical phase shifter to set an optical phase at a particular point of time to a fixed value using a pattern sync signal as a reference. An optical input sampling oscilloscope repeatedly averages optical waveforms and a CPU conducts three-dimensional display of the optical electric field waveform from which noise has been removed.