  We report the discovery of superconductivity in the ternary aluminide
Nb$_{5}$Sn$_{2}$Al, which crystallizes in the W$_{5}$Si$_{3}$-type structure
with one-dimensional Nb chains along the $c$-axis. It is found that the
compound has a multiband nature and becomes a weakly coupled, type-II
superconductor below 2.0 K. The bulk nature of superconductivity is confirmed
by the specific heat jump, whose temperature dependence shows apparent
deviation from a single isotropic gap behavior. The lower and upper critical
fields are estimated to be 2.0 mT and 0.3 T, respectively. From these values,
we derive the penetration depth, coherence length and Ginzburg-Landau parameter
to be 516 nm, 32.8 nm and 15.6, respectively. By contrast, the isostructural
compound Ti$_{5}$Sn$_{2}$Al dose not superconduct above 0.5 K. A comparison of
these results with other W$_{5}$Si$_{3}$-type superconductors suggests that
$T_{\rm c}$ of these compounds correlates with the average number of valence
electrons per atom.
