A switchover behavior between long-term potentiation (LTP) and long-term depression (LTD) characteristics in an amorphous Ga-Sn-O (a-GTO) thin-film spike-timing-dependent-plasticity (STDP) device has been observed. The LTP characteristic is induced by applying short-width pulses, whereas the LTP and LTD hybrid characteristic is induced by applying long-width pulses. This is suggested to be due to the difference between drift velocities of oxygen ions and vacancies. The switchover behavior may be useful for general operations of neuromorphic systems.