Abstract: This article has described in detail how the special generating top chopping (or engaging interference) is caused in cutting (or designing) the internal gears, and its testing method. This method is more accurate and concrete than the approximation process (1),(2) used nowadays. As the mathematical approach is very simple, it is easy to understand and master. Moreover, in order to speed up checking, a set of checking program has been worked out, which is suitable for the programmed Calculator CASIO FX-702P. In this way, it only takes a minute or less to finish the whole calculation. So it is of considerably theoretical as well as practical value.