We report a measurement of muon-neutrino disappearance in the T2K experiment. The 295-km muon-neutrino beam from Tokai to Kamioka is the first implementation of the off-axis technique in a long-baseline neutrino oscillation experiment. With data corresponding to 1.43×1020 protons on target, we observe 31 fully-contained single μ-like ring events in Super-Kamiokande, compared with an expectation of 104±14 (syst) events without neutrino oscillations. The best-fit point for two-flavor νμ→ντ oscillations is sin2(2θ23)=0.98 and |Δm232|=2.65×10−3  eV2. The boundary of the 90% confidence region includes the points (sin2(2θ23),|Δm232|)=(1.0,3.1×10−3  eV2), (0.84, 2.65×10−3  eV2) and (1.0, 2.2×10−3  eV2).