To improve the graininess of electrophotographic images, it is important to determine not only graininess values but also the root cause of graininess deterioration so as to provide feedback for use in electrophotographic image system development. In a previous report [1], a graininess separation method for clarifying the cause of graininess deterioration and its use to optimize an electrophotographic system was reported. It was clarified that using the method improved image processing, achieved better graininess, and made system development more effective. The method separates image graininess into 1) lightness fluctuation in background areas, 2) lightness fluctuation in dotted areas, and 3) dot size fluctuation. These factors are the root cause of image graininess deterioration. Therefore, being able to clarify the effect each factor has on graininess makes it possible to improve electrographic systems easily.. This report describes a graininess separation devised for secondary images and examines its validity. The method makes it possible to identify the cause of graininess degradation merely by taking a picture of an arbitrary secondary colour image. It also makes it possible to assume what colours need to be improved and what processing of the system need to be optimized. By using this method, electrophotpgrapic system can be optimized efficiently.