  Superconducting transition under pressure ($P$) has been investigated for
optimum-doped $R$FeAsO$_{\rm 1-x}$F$_{\rm x}$ ($R$=La, Ce$-$Sm) by dc
magnetization measurements. For $R$=La, $T_{\rm c}$ is found to be pressure
independent up to $P$$\sim$3.0 GPa and then shows a monotonic decrease with
increasing pressure. The plateau width decreases for the system with smaller
lattice constants, and shrinks to almost zero for $R$=Sm. From the $T_{\rm
c}$($P$) data, we construct the $T_{\rm c}$ evolution map on the height of the
As atom from the Fe-plane $h_{\rm As}$ versus lattice constant $a$ or $c$
plane. It is shown that the characteristic $T_{\rm c}$ variations under
pressure for all compounds and the rapid decrease in $T_{\rm c}$ induced by
changing $R$-element from Sm to La can be described by a common $T_{\rm
c}$($h_{\rm As}$, $a$ or $c$) surface, suggesting that $T_{\rm c}$ is
determined by $h_{\rm As}$ and lattice constant. It is also suggested that
there exists an upper limit of the lattice constant $a_{\rm ulm}$ (or $c_{\rm
ulm}$), above which $dT_{\rm c}$$da$ (or $dT_{\rm c}$$dc$) changes the sign
from positive to negative.
