A corrosion evaluation device for performing a quantitative evaluation of corrosion by measuring a quantity of decrease in the thickness of a magnetic material which is covered by a non magnetic material or a magnetic material which is not covered by a non magnetic material which includes: a magnetic field generating device for generating such a magnetic field that includes the magnetic material in a magnetic path, a Giant Magnet-Resistive effect (GMR) sensor provided with a GMR sensor for detecting a magnetic flux leakage with regard to the magnetic material and converts a change in the magnetic flux into an electrical signal, a thickness reduction calculation portion for calculating a quantity of decrease in the thickness of the magnetic material based on the electrical signal. The corrosion evaluation device precisely performs a quantitative evaluation of corrosion even when the magnetic material, which is to be evaluated, is covered by a non magnetic material.