  We study the origin of the large thermopower in quasi-two-dimensional a
$\tau$-type organic conductor, $\tau-(EDO-S,S-DMEDT-TTF)_2(AuBr_2)_{1+y}$ ($y
\le 0.875$), from the view point of a "pudding mold"-type band structure. We
calculate the electronic band structure using an \textit{ab initio} band
calculation package, and obtain a tight binding model fit to the \textit{ab
initio} band structure. Using the model and the Boltzmann's equation approach,
we calculate the temperature dependence of the Seebeck coefficient. We conclude
that the peculiar band structure is the origin of the large Seebeck coefficient
and the appearance of the maximum value at a certain temperature.
