  We have measured the branching fraction B, longitudinal polarization fraction
f_L, and CP asymmetry coefficients A and S for B0->rho+rho- decays with the
Belle detector at the KEKB e+e- collider using 253 fb-1 of data. We obtain B =
[22.8 +-3.8 (stat) +2.3 -2.6 (syst) ] x 10-6, f_L = 0.941 +0.034 -0.040 (stat)
+-0.030 (syst), A = 0.00 +-0.30 (stat) +-0.09 (syst), and S = 0.08 +-0.41
(stat) +-0.09 (syst). These values are used to constrain the
Cabibbo-Kobayashi-Maskawa phase phi_2; the solution consistent with the
Standard Model is phi_2 = (88 +-17) degrees or 59 deg.< phi_2 < 115 deg. at 90%
CL.
