Simultaneous measuring system of tunneling current and displacement current has been developed for direct detecting the single electron motion in nanomechanical single electron devices. Tunneling current and displacement current staircases have been observed simultaneously in current-voltage characteristics of the double barrier tunneling junctions (DBTJs) that consist of scanning vibrating probe/vacuum/colloidal Au nanoparticles/Au(111) substrate. Single electron motion on and through the colloidal Au nanoparticles has been analyzed by both the displacement current and tunneling current staircases. The negative differential conductance has also been observed in the current-voltage dependence of the same DBTJs. For realizing the nanomechanical single electron device with self-excitation, the utilization of alkanethiol molecules as the tunneling barrier has been discussed.