A corrosion degree measuring method capable of field measurement for intergranular corrosion, particularly the sensitization degree, of the plant's structural materials in a narrow space. The metal electrode (24) made of a material being measured and the counter electrode (17) to the metal electrode are immersed in an electrolytic solution. A voltage is applied to the metal electrode (24) in the anodic direction so as to raise the potential of the metal electrode (24) up to the passive state potential and to keep the metal electrode (24) at the passive state potential. The passive state potential is used as a reference potential, and a pulse-like potential signal is applied to the metal electrode in the cathodic direction. At the time of the application of the pulse potential signal, a current between the metal electrode (24) and the counter electrode (17) is measured, and the corrosion degree of the material being measured is determined from the intensity of the current.