We present results for ν_μ oscillation in the KEK to Kamioka (K2K) long-baseline neutrino oscillation experiment. K2K uses an accelerator-produced ν_μ beam with a mean energy of 1.3 GeV directed at the Super-Kamiokande detector. We observed the energy dependent disappearance of ν_μ, which we presume have oscillated to ν_τ. The probability that we would observe these results if there is no neutrino oscillation is 0.0050% (4.0σ).