  Magnetic excitations of the filled skutterudite PrRu4P12 exhibiting a
metal-insulator (M-I) transition at TM-I= 63 K were studied by inelastic
neutron scattering experiment. The spectra at temperatures much lower than TM-I
are described as well-defined crystal-field excitations. With approaching TM-I,
the excitation peaks broaden and shift considerably together with the
temperature variation of the carrier number and the atomic displacement in the
transition. The 4f-electron state evolve from the well localized state in the
insulator phase to the strongly hybridized itinerant state by p-f mixing near
TM-I. The hybridization is responsible for the M-I transition of PrRu4P12.
