Dendrimers are nanometer size macromolecules with a regular tree-like array of branch units. For these macromolecules, it has been confirmed by various experiments that the phonon energy which is supplied by the infrared light irradiation at the periphery of these molecules migrates to the core efficiently. In order to clarify such an anisotropic migration of phonons in the dendrimer, we investigate the dynamical natures of the phonon in this molecule using a classical oscillator model which approximates complicated vibrations of the dendrimer as simple oscillators. We calculate the time developments of migration of photoexcited phonons solving the Newton equation for the classical oscillator model. From our calculated results, it is clarified that the phonon has a tendency to migrate efficiently from the outside to the core of this molecule through anharmonic vibrations between composition units of the dendrimer, and its energy is trapped in the inner region for a long time. We conclude that the characteristic tree-like structure of the dendrimer, wherein the composition units crowd toward the outside from the inside, plays an essential role in the anisotropic migration of phonons.