A total of 614 upward throughgoing muons of minimum energy 1.6 GeV are observed by Super-Kamiokande during 537 detector live days. The measured muon flux is [1.74±0.07(stat)±0.02(sys)]×10-13cm-2s-1sr-1 compared to an expected flux of [1.97±0.44(theor)]×10-13cm-2s-1sr-1. The absolute measured flux is in agreement with the prediction within the errors. However, the zenith-angle dependence of the observed upward throughgoing muon flux does not agree with no-oscillation predictions. The observed distortion in shape is consistent with the vμ⟷vτ oscillation hypothesis with sin22θ> 0.4 and 1×10-3Δm2<1×10-1eV2 at 90% confidence level.