To more surely suppress corrosion of a component by detecting electrochemical phenomena of the component with excellent precision in a nuclear reactor component and a piping contacting to reactor water or cooling water.

An electrochemical sensor according to this method consists of a reference electrode and a test specimen, and reference electrodes 9 and test specimens 10 made of the same material to the component are arranged in a reactor core part 2, a primary loop recirculation system, and a feed water line. Potential difference between the reference electrode 9 and the test specimen 10 is measured to detect the corrosive environment based on excessive voltage change in the test specimen 10.