To investigate the mechanism of recently observed silent slip events, we simulated earthquake preparation processes using the Dieterich/Ruina rate- and state-dependent friction law. To ensure realistic modeling of the unstable-stable transition, we considered small cut-off velocity to an evolution effect in the friction law for the transition zone. When the cut-off velocity to the evolution effect is significantly smaller than that of a direct effect, steady state friction behaves as velocity weakening at low slip velocity and velocity strengthening at high slip velocity. This frictional behavior was experimentally and theoretically confirmed for the unstable-stable transition zone. The results of our numerical simulations show that silent slips of which velocity is higher than the velocity of relative plate motion, eventually propagates horizontally along the unstable-stable transition over a period of several years. Silent slip events can be interpreted as being caused by the transitional behavior of the fault constitutive law.