PURPOSE: To make it possible to judge corrosion over broad frequencies in a short time, by measuring the response of a circuit network with respect to a stimulating signal including various frequency components, performing Frourier transforming process, and computing the transmission function of each frequency.

CONSTITUTION: When a switch 5 is switched, electric charge in a capacitor 1 is applied to a reference electrode 10, an opposed electrode 3 and a sample metal piece 2 under the conducting state by a corrosive liquid 4. The pulse current including various frequency components is converted into a voltage by a variable resistor 8. The voltage is supplied to a high speed Fourier transform device 13 through a differential amplifier 9. The response potential of the metal piece 2 is supplied to the device 13 through a differential amplifier 11 with the electrode 10 as a reference. Based on the measurement of one response, the device 13 computes the impedance of the metal piece for each frequency and the phase difference by the Fourier transform. The transmission function over the broad frequencies is determined in a short time, and the corrosion state is judged.