PURPOSE: To eliminate the need for a costly high speed A/D converter of multibits and lower the cost of production by obtaining the digital value used for calculation of average values by low speed A/D conversion.

CONSTITUTION: The output of a low speed A/D converter 61 inputted with an analog signal (a) is inputted to an average level operating circuit 52 consisting of plural registers and adders, and a digital average signal (b') is latched by latch circuits 63, 64. The digital average signal (f') thereof is again converted to an analog signal (f) by a low speed D/A converter 65, and the difference between the analog signal (f) and the analog signal (a) is amplified by a differential amplifier 66. This signal (g) is converted by a high speed A/D converter 67, by which a digital signal (g') for flaw detection is obtained. Thereby, inexpensive circuits are used and the production cost for the device is lowered.