In a method of reducing corrosion of a material constituting a nuclear reactor structure, an electrochemical corrosion potential is controlled by injecting a solution or a suspension containing a substance generating an excitation current by an action of at least one of radiation, light, and heat existing in a nuclear reactor, or a metal or a metallic compound forming the substance generating the excitation current under the condition in the nuclear reactor to allow the substance generating the excitation current to adhere to the surface of the nuclear reactor structural material, and by injecting hydrogen in cooling water of the nuclear reactor while controlling the hydrogen concentration in a feed water.