An electro-optic sampling prober is used to measure a waveform of a measured signal applied to wiring of an IC wafer. Herein, a laser radiates laser beams, which are supplied to an optical module containing an optical isolator and photodiodes by way of an optical fiber. Then, the laser beams pass through an optical wavelength filter to propagate through a prober unit. The laser beams are incident on an electro-optical element, which is changed in polarization state in response to an electric field being caused by the measured signal. The laser beams are reflected by a surface mirror of the electro-optical element, so that reflected beams propagate back through the prober unit and are returned to the optical module by way of the optical wavelength filter. During the measurement, a human operator watches an image of a selected portion of the IC wafer presently placed beneath the prober unit to adjust a positional relationship between the prober unit and IC wafer. The image is produced by an infrared camera equipped with a halogen lamp and a monitor. Incidentally, the optical wavelength filter has an optical characteristic such that a center wavelength in transmission of light coincides with a wavelength of the laser beams whose intensities are maximal, so it is possible to prevent components of light, which are not required for measurement, from being unnecessarily returned to the optical module, and it is possible to improve a S/N ratio in measurement.