  We report measurements of time-dependent decay rates for $B^0 \to D^{*\mp}
\pi^\pm$ decays and extraction of CP violation parameters related to $\phi_3$.
We use a partial reconstruction technique, whereby signal events are identified
using information only from the primary pion and the charged pion from the
decay of the $D^{*\mp}$. The analysis uses $140 {\rm fb}^{-1}$ of data
accumulated at the $\Upsilon(4S)$ resonance with the Belle detector at the KEKB
asymmetric-energy $e^{+}e^{-}$ collider. We measure the CP violation parameters
$S^+ = 0.035 \pm 0.041 ({\rm stat}) \pm 0.018 ({\rm syst})$ and $S^- = 0.025
\pm 0.041 ({\rm stat}) \pm 0.018 ({\rm syst})$.
