A detector detects an electromagnetic wave having a frequency of 0.01 THz≦f≦100 THz and transmitted through a device under test (DUT). A changer changes a relative position of an intersection of an optical path of the electromagnetic wave and the DUT, with respect to the DUT. A deriver derives a characteristic value of the electromagnetic wave based on a detection result of the detector, while the characteristic value is associated with an assumed relative position, which is the relative position if the electromagnetic wave is not refracted by the DUT. A corrector changes the assumed relative position to an actual relative position, which is the relative position if the refraction of the electromagnetic wave by the DUT is considered. A corrected deriver derives the characteristic value associated with a predetermined relative position based on an output from the corrector.