PURPOSE: To improve position resolution up to micron order by projecting a converged light beam on a sample and detecting variation of generated optically induced electromotive force due to ESR (electron spin resonance) absorption.

CONSTITUTION: A sample 5 can be fixed between a light transmitting electrode 3 and an electrode and sample base 4 in a cavity resonator 1 connected to an ESR spectrometer through a waveguide 2. A light beam 6 which is converged from a light irradiation window 7 in the flank of the cavity resonator 1 is moved and scanned on the sample 5 to modulate a magnetic field by a modulating coil 9 within a range where electron spin resonance conditions vary greatly. The variation of the optically induced electromotive force synchronized with the modulation is detected to evaluate the distribution of specific defects in the sample 5 with position resolution of micron order.