PURPOSE: To nondestructively find the thickness of a contaminated layer produced near the surface of a cast article due to the mold used for manufacturing the cast article by casting a metal.

CONSTITUTION: The concentration of an impurity element contained in a reaction layer 14 produced at the boundary between a mold 10 and a molten metal 12 as a result of a reaction is found by calculating the amount of the impurity element contained in the layer 14 and, at the same time, a concentration formula is found by resolving a diffusion equation by setting the condition of a diffusing environment, such as the cooling speed of a cast article, initial state of the impurity element, etc. By integrating the concentration formula with respect to the distance (depth from the surface), finding a recurrence formula regarding the time expressing the concentration of the impurity about the surface and other part other than the surface, and finding a concentration profile from the surface by using the recurrent formula, the thickness of the reaction layer 14 near the surface of the cast article is decided based on the concentration profile.