PURPOSE: To detect the spatial high-order differentiation of a body to be measured, by obtaining the difference among the outputs of plural detectors for performing primary differentiation.

CONSTITUTION: Three detectors 12a, 12b and 12c are arranged along the running direction of a body 11 to be detected. The outputs of the 1st and 3rd detectors 12a and 12c are added together by an adder 13, and the output of the 2nd detector 12b is multipled by two through an amplifier 14; and they are applied to the positive and negative input terminals of a differential amplifier 15. The output signal of the differential amplifier 15 has a secondary differential waveform.