  We report results on time-dependent CP asymmetries in $B \to
D^{*\mp}\pi^{\pm}$ decays based on a data sample containing 657 {\times} $10^6$
$B\bar{B}$ pairs collected with the Belle detector at the KEKB
asymmetric-energy $e^+ e^-$ collider at the $\Upsilon(4S)$ resonance. We use a
partial reconstruction technique, wherein signal $B \to D^{*\mp}\pi^{\pm}$
events are identified using information only from the fast pion from the B
decay and the slow pion from the subsequent decay of the $D^{*\mp}$, where the
former (latter) corresponds to $D^{*+} (D^{*-})$ final states. We obtain CP
violation parameters $S^+ = +0.061 \pm 0.018(\mathrm{stat}) \pm
0.012(\mathrm{syst})$ and $S^- = +0.031 \pm 0.019(\mathrm{stat}) \pm
0.015(\mathrm{syst})$.
