  We synthesized single crystals of the new hexagonal compounds
A$_2$Cu$_3$SnF$_{12}$ with A=Cs and Rb, and investigated their magnetic
properties. These compounds are composed of Kagom\'{e} layers of corner-sharing
CuF$_6$-octahedra. Cs$_2$Cu$_3$SnF$_{12}$ has the proper Kagom\'{e} layer at
room temperature, and undergoes structural phase transition at
$T_\mathrm{t}\simeq 185$ K. The temperature dependence of the magnetic
susceptibility in Cs$_2$Cu$_3$SnF$_{12}$ agrees well with the result of the
numerical calculation for $S=1/2$ two-dimensional Heisenberg Kagom\'{e}
antiferromagnet down to $T_\mathrm{t}$ with the nearest exchange interaction
$J/k_\mathrm{B}\simeq 240$ K. Although the magnetic susceptibility deviates
from the calculated result below $T<T_\mathrm{t}$, the rounded maxima were
observed at approximately $T\simeq (1/6)J/k_\mathrm{B}$ as predicted by the
theory. Cs$_2$Cu$_3$SnF$_{12}$ undergoes three-dimensional magnetic ordering at
$T_\mathrm{N} = 20$ K.
