A self-consistent particle simulation with a dipole magnetic field model is carried out, reproducing chorus emissions with rising tones successfully. We assume energetic electrons forming a highly anisotropic velocity distribution in the equatorial region. No initial wave is assumed except for electromagnetic thermal noise induced by the energetic electrons. In the early stage of the simulation, coherent whistler-mode waves are generated from the equator through an instability driven by the temperature anisotropy of the energetic electrons. During the propagation of the whistler-mode waves, we find formation of a narrowband emission with negative frequency gradient (NEWNFG) in the spatial distribution of the frequency spectrum in the simulation system. The trailing edge of NEWNFG is continuously created at increasing frequencies in the region close to the equator. Observed at a fixed point, the NEWNFG shows a frequency variation of a typical chorus emission.