PURPOSE: To provide a light sampling device for compensating irregularity of a wave surface of reflected light caused by a minor variation of a reflected surface or to dispositioning of an optical system.

CONSTITUTION: A light sampling device in which the condition of a plane of polarization of an optical pulse is changed according to the operational voltage of a circuit to be measured, and the voltage waveform of the circuit to be measured is measured by detecting the change in the plane of polarization, comprises a light pulse formation means 51 and an electric optical element for changing the plane of polarization of incident light according to an electric field corresponding to the operational voltage of the circuit to be measured. A wave surface compensation means 54 for compensating the wave surface by a phase conjugate wave, a forking means 52 for forking an optical signal having a specific plane of polarization, a light receiving element 3 for receiving the forked light, and a calculation process means 53 for calculating the voltage waveform of the circuit to be measured based on the output of the light receiving element 3, are also provided.