PURPOSE: To detect a surface defect with good accuracy by specifying the range of the frequency of ultrasonic waves and the range of the angle of incidence.

CONSTITUTION: A varistor 1 is a sample, and a defect of a polishing surface 1a of the varistor is to be inspected. In other words, when the ultrasonic waves are projected to the polishing surface 1a by a probe 2, the surface waves generated at the polishing surface 1a are picked up and the amount of echoes is detected by an oscillograph. At this time, if there is a defect such as a pin hole or the like, defect echoes are generated and the level of back noise echoes due to the influences of crystal particles etc., at the polishing surface 1a is raised. Based on a suitable condition to distinguish both echoes, a favorable detection is achieved in the frequency range 3-7 MHz from the relationship between the frequency of ultrasonic waves and the ratio of the amount of defect echoes and that of noise echoes. Then, it is found that the angle of incidence is suitable in the range 30-40° from the relationship between the angle of incidence and the amount of echoes. That is, if the frequency and the angle of incidence are set, in the above respective ranges, the surface defect can be detected accurately.