A wafer is irradiated with laser light having a wavelength which is transmitted through an inside of a crystal of the wafer and does not generate an electromotive force due to photo-excitation while the laser light is scanned. When a temperature of the wafer is increased by the irradiation, a thermo-electromotive force is generated in a crystalline abnormal part of the wafer by a Seebeck effect. A defect inside the crystal is detected such that the thermo-electromotive force is detected by a change in voltage or current which appears between an anode and a cathode of the wafer and the thermo-electromotive force is displayed on a CRT.