One part of the vacuum vessel (the plasma facing material) of the fusion experimental device and future fusion reactor comes into contact with plasma. When the plasma ions enter into the material, those particles become a neutral atom and stay inside the material. If seen from the atoms that compose the material, the plasma ions that entered become impurity atoms. The impurity atoms migrate slowly in interspaces among the atoms that composes the material and eventually, they diffuse inside the material. On the other hand, some impurity atoms return to the surface and are again emitted to the plasma. For the stable confinement of fusion plasma, the balance between the penetration of plasma ions into the material and the re-emission of impurity atoms after migration from inside the material becomes extremely important.