  We study the parametric resonance of the neutrino oscillation through the
matter whose density varies spatially. The Fourier analysis of the matter
effect enables us to clarify the parametric resonance condition, which is
summarized in a frequency matching between the neutrino oscillation and the
spatial variation of the matter density. As a result, the n-th Fourier mode of
a matter density profile modifies the energy spectrum of the nu_mu -> nu_e
appearance probability at around the n-th dip.
