  The electric conductivity of Dirac electrons in the organic conductor
$\alpha$-STF$_2$I$_3$ (STF = bis(ethylenedithio)diselenadithiafulvalene), which
has an isostructure of ET(=bis(ethylenedithio)tetrathiafulvalene), has been
theoretically studied using a two-dimensional tight-binding model in the
presence of both impurity and electron-phonon (e-p) scatterings.In contrast to
ET, which has a Dirac cone with almost isotropic velocity, STF provides a large
anisotropy owing to a Dirac point that is close to merging. As a result,
$\sigma_{x}$ becomes much larger than $\sigma_{y}$, where $\sigma_y$ and
$\sigma_x$ are diagonal conductivities parallel and perpendicular to a stacking
axis of molecules, respectively. With increasing temperature ($T$), $\sigma_x$
takes a broad maximum because of e-p scattering and $\sigma_y$ remains almost
constant. The ratio $\sigma_x/\sigma_y$ is analyzed in terms of the band
structure. Such an exotic conductivity of STF is compared with that of an
experiment showing a good correspondence. Finally, $\sigma_x/\sigma_y$ values
of ET and BETS(= bis(ethylenedithio)tetraselenafulvalene) are shown to
demonstrate the dissimilarity with STF.
