Details of corrosion process under the coating layer of the steel sheet have not been understood well. Therefore, it is required to develop a nondestructive analytical method for the near surface of the steel sheet. It is also required to have analytical tool for observing the corrosion process in the solution. The authors developed a confocal 3D-XRF analytical instrument, which enables a nondestructive elemental analysis near the surface of the material. This technique was applied for analyzing the corrosion process under the coating layer of the steel sheet. Depth elemental images showing elemental distributions were nondestructively obtained. 3D-XRF method was also applied for observing Fe distribution in the NaCl solution during corrosion of Fe from the steel sheet. Dissolution, migration, and deposition processes were successfully monitored. It is expected that this confocal 3D-XRF technique will be applied for in-situ analysis of the steels under various environments.