[en] In the recycling process of impurities in a plasma, only the sputtering of first wall materials is usually considered. In the present study, not only sputtering of metal but desorption of sorbed layers by particles and reflection of incident impurity ions at the limiter are also introduced. The behaviour of impurities in the discharges of recent Tokamaks is interpreted quantitatively and the result is applied to a future large Tokamak. The nonmetallic impurity concentrations may attain plateaus in the early stage of a discharge which are lower than the permissible levels, because the desorption yield by particles may probably be decreased by discharge cleaning. On the contrary the metallic impurity concentrations tend to increase beyond the permissible levels, since the self-sputtering yield of impurity ions will become larger than unity with the increase of plasma temperatures. In this respect the method to reduce the metallic impurities is important. (auth.)