It is clarified by the experimental procedure and theoretical models that there exist two types in toner concentration dependence of toner charge at two-component developer. One is independent (Type 1) and other is dependent (Type 2) on toner concentration. The reason of these phenomena is responsible to the relative difference in each charging site number of toner and carrier of given developer, and is not the contamination of carrier surfaces by the toner. New model and also conventional models will explain these phenomena by introduction of the relative difference of the charging site number of toner and carrier. The characteristics of the two-component developer can be classified as Type 1 and Type 2 for the developer design.