PURPOSE: To accurately evaluate the corrosion of an actual metal or non-metal structure surface by forming a flexible electrode on a non-conductive flexible thin layer mountable on the surface of a measurement object, and detecting an electrical signal generated across electrodes due to the corrosion.

CONSTITUTION: Electrodes 2 to 4 are mounted on the non-conductive flexible thin layer 18 of polyimide having thickness approximately between 150 and 250μm at intervals 5 of approximately 0.25mm. thereby forming a sensor 1. Then, the sensor 1 is bonded to the surface of a measured panel 21. An electrical signal generated across the electrodes 2 and 3 due to the surface corrosion of the panel 21 is, then, inputted to an electrochemical potential noise monitor device 6, while an electrical signal generated across the electrodes 3 and 4, and 2 is inputted to an electrochemical current noise device 10. Output signals 7 and 11 from the devices 6 and 10 are compared with each other in a circuit 12, and when the ratio of the signals 7 and 11 has a small value, a signal showing overall corrosion is outputted at a step 15. Also, when the ratio has a large value, a signal showing local corrosion is outputted at the step 15. As a result, the characteristic of a special corrosion process and a corrosion ratio can be accurately evaluated.