Evidence of muon neutrino oscillation with an accelerator-based experiment is reported. Data corresponding to 8.91 x 10~(19) p.o.t. were used for the analysis. There were 108 events fully contained in the Super-Kamiokande inner detector fiducial area, which were synchronized to the beam-spill timing. In the case of no oscillations, the expected number of events was 150.9_(-10.0)~(+11.6). Out of the 108 events, we obtained 56 events of single ring μ-like events. The neutrino energy spectrum, reconstructed by assuming two-body kinematics of quasi-elastic interactions, shows a clear oscillation pattern compared with the spectrum at the production observed by the near detectors. These two facts strongly indicate neutrino oscillation with common oscillation parameter regions. A combined oscillation analysis gives Δm~2 = 1.7 ～ 3.5 x 10~(-3) eV~2 at sin~2 2θ = 1.0 at the 90% confidence level, and the null oscillation probability is found to be less than 0.01%.
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