  We report a measurement of CP asymmetry parameters in the $b \to
c\bar{c}d$-transition-induced decays $B^0(\bar{B^0}) \to J/\psi pi^0$. The
analysis is based on a 140 fb$^{-1}$ data sample accumulated at the
$\Upsilon(4S)$ resonance by the Belle detector at the KEKB asymmetric-energy
$e^+ e^-$ collider. We fully reconstruct one neutral $B$ meson in the $J/\psi
~\pi^0$ final state. The accompanying $B$ meson flavor is identified by its
decay products. From the distribution of proper time intervals between the two
B decays, we obtain the following CP violation parameters: S(J/\psi \pi^0)=
-0.72 \pm0.42 (stat) \pm 0.08(syst) A(J/\psi \pi^0)= -0.01 \pm0.29 (stat) \pm
0.07(syst).
