  In order to explore the reason why the single-layered cuprates,
La$_{2-x}$(Sr/Ba)$_x$CuO$_4$ ($T_c\simeq$ 40K) and HgBa$_2$CuO$_{4+\delta}$
($T_c\simeq$ 90K), have such a significant difference in $T_c$, we study a
two-orbital model that incorporates the $d_{z^2}$ orbital on top of the
$d_{x^2-y^2}$ orbital. It is found, with the fluctuation exchange
approximation, that the $d_{z^2}$ orbital contribution to the Fermi surface,
which is stronger in the La system, works against d-wave superconductivity,
thereby dominating over the effect of the Fermi surface shape. The result
resolves the long-standing contradiction between the theoretical results on
Hubbard-type models and the experimental material dependence of $T_c$ in the
cuprates.
