  In order to propose a ferromagnet exhibiting highly spin-polarized transport,
we theoretically analyzed the spin polarization ratio of the conductivity of
the bulk Fe$_4$N with a perovskite type structure, in which N is located at the
body center position of fcc-Fe. The spin polarization ratio is defined by $P =
(\sigma_\uparrow - \sigma_\downarrow) / (\sigma_\uparrow + \sigma_\downarrow
)$, with $\sigma_{\uparrow(\downarrow)}$ being the conductivity at zero
temperature of the up spin (down spin). The conductivity is obtained by using
the Kubo formula and the Slater-Koster tight binding model, where parameters
are determined from the least-square fitting of the dispersion curves by the
tight binding model to those by the first principles calculation. In the
vicinity of the Fermi energy, $|P|$ takes almost 1.0, indicating perfectly
spin-polarized transport. In addition, by comparing Fe$_4$N to fcc-Fe
(Fe$_4$N$_0$) in the ferromagnetic state with the equilibrium lattice constant
of Fe$_4$N, it is shown that the non-magnetic atom N plays an important role in
increasing $|P|$.
