  Anomalous metal-insulator transition observed in filled skutterudite
CeOs$_4$Sb$_{12}$ is investigated by constructing the effective tight-binding
model with the Coulomb repulsion between f electrons. By using the mean field
approximation, magnetic susceptibilities are calculated and the phase diagram
is obtained. When the band structure has a semimetallic character with small
electron and hole pockets at $\Gamma$ and H points, a spin density wave
transition with the ordering vector $\mathbf{Q}=(1,0,0)$ occurs due to the
nesting property of the Fermi surfaces. Magnetic field enhances this phase in
accord with the experiments.
