  We report the first measurement of the $D^{\ast -}$ meson polarization in the
decay $B^0 \to D^{*-} \tau^+\nu_{\tau}$ using the full data sample of
772$\times 10^6$ $B\bar{B}$ pairs recorded with the Belle detector at the KEKB
electron-positron collider. Our result, $F_L^{D^\ast} = 0.60 \pm 0.08 ({\rm
stat}) \pm 0.04 ({\rm sys})$, where $F_L^{D^\ast}$ denotes the $D^{\ast-}$
meson longitudinal polarization fraction, agrees within about $1.7$ standard
deviations of the standard model prediction.
