Electromagnetic field distribution is measured by considering time variations of a measured electric and/or magnetic field value. In response to scanning performed with a probe at an arbitrary set of measurement coordinates in a predetermined measurement plane in the vicinity of an object to be measured, a signal is detected at each of plural sets of coordinates in a measurement plane. Electric and magnetic fields are computed during a measurement time period at each set of measurement coordinates in the measurement plane based on measurement coordinates where the probe is positioned and the signal is detected with the probe. Amplitude probability distribution during the time period at each set of measurement coordinates in the vicinity of the object is computed based on the computed intensity, then mapped and displayed.