  The Seebeck coefficient, which is proportional to a ratio of the
thermoelectric conductivity to electrical conductivity has been examined for
Dirac electrons in the organic conductor $\alpha$-(BEDT-TTF)$_2$I$_3$ [BEDT-TTF
denotes a molecule given by bis(ethylenedithio)tetrathiafulvalene] under a
uniaxial pressure using a two-dimensional tight-binding model with both
impurity and electron--phonon (e--p) scatterings. We calculate an anomalous
temperature ($T$) dependence of the Seebeck coefficient $S_{\nu}$ with $\nu =
x$ (perpendicular to the molecular stacking axis) and $y$, which shows $S_\nu >
0$ with a maximum at high temperatures and $S_{\nu} < 0$ with a minimum at low
temperatures.The microscopic mechanism of such a sign change of $S_\nu$ is
clarified in terms of the spectral conductivity. The result is compared with
experiments on $\alpha$-(BEDT-TTF)$_2$I$_3$.
