  We present a measurement of CP asymmetry using a time-dependent Dalitz plot
analysis of B0 --> pi+pi-pi0 decays based on a 414/fb data sample containing
449M BB pairs. The data was collected on the $\Upsilon$(4S) resonance with the
Belle detector at the KEKB asymmetric energy e+e- collider. Combining our
analysis with information on charged B decay modes, we perform a full Dalitz
and isospin analysis and obtain a constraint on the CKM angle $\phi_2$, 68 deg.
< $\phi_2$ < 95 deg. as the 68.3% confidence interval for the $\phi_2$ solution
consistent with the standard model (SM). A large SM-disfavored region also
remains.
