PURPOSE: To perform the measurement automatically and quickly, by selecting automatically a linear part from the change with time of a measured overvoltage to calculate a corrosion rate when the corrosion rate is measured in accordance with the coulostat method.

CONSTITUTION: The overvoltage of a sample metal S to which electric charge is given from an electric charge giving mechanism (a) is detected by an overvoltage detecting means (c) and is stored in a storage means (d) successively for a prescribed time. Thereafter, overvoltage values in the storage means (d) are taken out in the reverse order from the last data by a taking-out means (e) and are transferred to a correlation coefficient calculating means (f). Each time a prescribed number of data are transferred, the correlation coefficient calculating means (f) performs the regression operation to output correlation coefficients successively to a maximum value detecting means (g). The maximum value detecting means (g) compares correlation coefficients with one another to detect a maximum value, and the overvoltage value group and the regression operation result which give the maximum correlation coefficient are transferred to a corrosion rate calculating means (h), and the corrosion rate is calculated in this means (h) on a basis of the output of a given electric charge quantity detecting means (b) and is displayed on a display means (i).