We present a measurement of the standard model CP violation parameter sin2
phi (1) based on a 29.1 fb(-1) data sample collected at the Y(4S) resonance
with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider. O
ne neutral B meson is fully reconstructed as a J/psiK(S), psi (2S)K-S, Xc1K
S, eta K-c(S), J/psiK(L), or J/psiK*(0) decay and the flavor of the accompa
nying 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 determine sin2 phi (1) = 0.99 +/-0.14(stat) +/-0.06(syst). We conclude
that we have observed CP violation in the neutral B meson system.