  We report the first measurement of CP-violation parameters in
  $B^0\to\rho^0\gamma$ decays based on a data sample of $657\times 10^6
B\bar{B}$ pairs collected with the Belle detector at the KEKB asymmetric-energy
$e^+ e^-$ collider. The flavor of the $B$ meson is identified from the decay
products of the accompanying $B$ meson. We obtain the time-dependent and direct
CP-violating parameters, $\cals_{\rhoz\gamma} = -0.83\pm 0.65(stat)\pm
0.18(syst)$ and $\cala_{\rhoz\gamma} = -0.44\pm 0.49(stat)\pm 0.14(syst)$,
respectively.
