PURPOSE: To determine automatically with a high accuracy a polarization curve having no discontinuous point by setting a test piece potential as prescribed, storing a polarization current which is brought to an electrochemical measurement, and operating an optimum current axis, etc. from the maximum value and the minimum value.

CONSTITUTION: A potential of a test piece 1 immersed in a corrosion liquid is set as prescribed through a D/A converter 10 controlled by a microcomputer 19 and a polarization use power source part 6. Subsequently, in accordance with an arrival to a measuring interval by a timer 12, the computer 19 controls a sensitivity signal generator 13 so that an output value of a current detecting part 4 becomes adequate, by reading a polarization current through the detecting part 4 and an A/D converter 11, and a read current value is stored in an RAM15. The same repeated measurement is executed by varying the set potential, and basing on the maximum value and the minimum value of the storage contents of the RAM15, the computer 19 operates an optimum current axis signal and a polarization curve signal, and supplies them to an X-Y recorder 9 through D/A converters 17, 18. According to this constitution, a polarization curve having no discontinuity due to switching of sensitivity is determined and drawn automatically.