PURPOSE:To remove a noise due to an irregular flow of a liquid, by leading an output of a potential detecting part into a low-pass filter which is capable of changing a cut-off frequency by a Fourier converter. CONSTITUTION:To a sample metallic piece 1 of an etching speed measuring cell 4 provided with the sample metallic piece 1 operating as a working electrode, a reference electrode 2 and an opposed electrode 3, charge of a capacitor 5 is supplied through the opposed electrode 3, and a variation of the polarized potential of the sample metallic piece 1 is detected by making the reference electrode a reference. This detecting signal is sent from a buffer amplifier 12 to a filter device 13 for attenuating a noise of a specific frequency component from the polarized potential, and a high speed Fourier converter 14, its noise component is removed, and a potential difference signal is recorded in a recorder 8. As for this filter device 13, a low-pass filter of 60Hz or below is effective against an induced noise from a commercial electric power source, but against a noise due to an irregular flow of a liquid, a cut-off frequency which exerts no influence on information of a polarized value is set by the high speed Fourier converter.
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