Scattered toner dust is a well-known problem in copying and printing machines using the electro photographic development process. The collection of toner dust by forming airflow in order to capture the dust with filters placed at fans was investigated. Specifically, system selections of airflow, which can effectively collect toner dust, were analyzed. In recent research, a conflict was found where forming airflow for collecting toner dust would generate more scattered toner dust. In other words, to avoid problems of such dirt in copy machines caused by toner dust, it is necessary to solve this conflict simultaneously. Therefore, a method for evaluating functionality of an airflow system was proposed. This method can evaluate the collection and restraint of toner dust with one index. By using this method, two airflow systems that have completely different concepts for collecting and suppressing toner scatter were evaluated. As a result, it was found that an airflow system with the concept of forming airflow clearly around developing units is better for collecting toner dust. Furthermore, it was clarified that this result is based on a mechanism of sedimentation adhesion of scattered toner by an influence of gravity.