In this paper, PN (Petri Net) is used for the qualitative modeling of SFC (Sequential Function Chart) rules of evolution with the cyclic scan of the programmable controller. With the proposed PN representation method, it is clarified that the SFC rules of evolution, such as the relationship between the active step and the applicable action, can be easily and systematically examined and analyzed, although the proposed PN is much complicated as compared with the general PN representation. Moreover, the relationship between the alternative and simultaneous sequences and the execution time of SFC is examined by the PN behavioral analysis. The experimental verification for the theoretical prediction shows that the execution time is independent of the number of the alternative sequences, but dependent on that, of the simultaneous ones.