  We present a measurement of the unitarity triangle angle phi_3 using a Dalitz
plot analysis of the three-body decay of the neutral D meson from the B->D(*)K
process. The method employs the interference between D0 and D0bar to extract
both the weak and strong phases. We apply this method to a 140 fb^{-1} data
sample collected by the Belle experiment. The analysis uses the modes B->DK and
B->D*K, D*->D pi0, where the neutral D meson decays into K_Spi+pi-. We obtain
146 signal candidates for B->DK and 39 candidates for B->D*K. From a combined
maximum likelihood fit to the B->DK and B->D*K modes, we obtain
phi_3=77^{\circ}^{+17}_{-19}(stat)+-13^{\circ}(syst)}+-11^{\circ}(model). The
corresponding two standard deviation interval is 26^{\circ}<phi_3<126^{\circ}.
