We carry out a self-consistent particle simulation with a dipole magnetic field to study the generation mechanism of VLF triggered emissions. The evolution of a wave packet propagating along a reference magnetic field line is solved by Maxwell's equations, while the bounce motion of energetic electrons .in the nonuniform magnetic field is taken into account. Simulation result demonstrates that our model can investigate the detailed physics of VLF triggered emissions. In the simulation result we find that a triggered emission with a rising tone is generated near the equatorial region after the wave packet passes through the magnetic equator, and 26% rising from the original frequency of the injected wave packet is reproduced. The generation process simulated in the present paper is explained by roles of resonant currents JE and JB in association with an electromagnetic electron hole in the phase space produced through the nonlinear interaction. It has been suggested that a negative JE contributes to the growth of whistler-mode waves, while a negative JB induces the frequency rising. Simulation result shows that both negative JE and negative JB are formed at the trailing edge of the triggering wave, generating the emission with the rising tone. On the basis of the simulation result, we find that these resonant currents are formed by untrapped electrons through the nonlinear wave-particle interaction and suggest that the resonant current formation by untrapped electrons plays an essential role in the triggering mechanism.