We numerically analyze the multiple Josephson junction (MJJ) model of high T C superconductors. MJJ applied a constant current has many hysteresis branches in I-V characteristics and generates oscillating voltage. We find that the hysteresis branches are originated from various modes of rotating phase differences between superconducting layers, and that the frequency characteristics depend on rotational velocity of each phase. In MJJ applied an alternating current we also show harmonizations of spontaneous and external oscillations.