  We present an improved measurement of the standard model CP violation
parameter sin2phi1 (also known as sin2beta) based on a sample of 45 x 10^6
BBbar pairs collected at the Upsilon(4S) resonance with the Belle detector at
the KEK asymmetric-energy e^+e^- collider. One neutral B meson is reconstructed
in a J/psi Ks, psi(2s) Ks, chi_c1 Ks, eta_c Ks, J/psi K*, or J/psi KL
CP-eigenstate decay channel and the flavor of accompanying B meson is
identified from its decay products. From the asymmetry in the distribution of
the time intervals between the two B meson decay points, we obtain sin2phi1 =
0.82 +/- 0.12(stat) +/- 0.05(syst).
