  Radiative transfer models were developed to understand the optical
polarizations in edge-on galaxies, which are observed to occur even outside the
geometrically thin dust disk, with a scale height of ~ 0.2 kpc. In order to
reproduce the vertically extended polarization structure, we find it is
essential to include a geometrically thick dust layer in the radiative transfer
model, in addition to the commonly-known thin dust layer. The models include
polarizations due to both dust scattering and dichroic extinction which is
responsible for the observed interstellar polarization in the Milky Way. We
also find that the polarization level is enhanced if the clumpiness of the
interstellar medium, and the dichroic extinction by vertical magnetic fields in
the outer regions of the dust lane are included in the radiative transfer
model. The predicted degree of polarization outside the dust lane was found to
be consistent with that (ranging from 1% to 4%) observed in NGC 891.
