The K2K, the KEK to Kamioka long-baseline neutrino oscillation experiment, observes neutrino oscillation; a reduction of muon neutrinos together with a distortion of the energy spectrum. One hundred and seven beam-originated neutrino events are observed in Super-Kamiokande in expectation of 151(-10)(+12) events without oscillation. The spectrum distortion by oscillation is also seen in fifty-seven single-ring muon-like events usable for energy reconstruction. The probability that the observations are explained by statistical fluctuation with no neutrino oscillation is 0.005 % (4.0 sigma). In a two flavor oscillation scenario, the allowed Delta m(2) region at sin(2) 2 theta=1 is between 1.9 and 3.6 x 10(-3) eV(2) at the go % C.L. with the central value of 2.8 x 10(-3) eV(2). The result of K2K confirms neutrino oscillation of atmospheric neutrinos reported by Super-Kamiokande collaboration.