A subtraction results value arrived at by subtracting a required subtraction value from a detection maximum value is taken as a maximum value in cases where a detection maximum value obtained from an RF signal is in a reducing direction, and an addition results value arrived at by adding a required value to a detection minimum value is taken as a minimum value in cases where a detection minimum value obtained from an RF signal is in an increasing direction. An RF amplitude value indicating an amplitude level of an RF signal is then obtained by subtracting the minimum value from the maximum value. Also, the subtraction value or the addition value is variably set according to the center value for the amplitude of the RF signal increasing or decreasing in excess of a prescribed level. As a result, accuracy of defect detection can be improved in cases where the amplitude level of the RF signal is biased on the maximum value side or the minimum value side.