Supersaturation dependence of crystal growth in the presence of impurity is discussed. The supersaturation dependence was divided conveniently into two types: the case where growth rate is suppressed over a wide range of supersaturation and the case where it is suppressed only in the low range of supersaturation while at the higher supersaturation range the impurity effect disappears completely. The latter case is explained theoretically by assuming slow unsteady state adsorption of impurity species at higher supersaturations. Literature data of the growth rate of an ammonium sulfate crystal are successfully explained. It is pointed out that classification of impurity action into the two types is not essential. The former type is a special case of the second one where the time constant of adsorption process becomes very small even at higher supersaturations.