We present a quenched lattice QCD calculation of the α and β parameters of the proton decay matrix element. The simulation is carried out using the Wilson quark action at three values of the lattice spacing in the range a ≈ 0.1 - 0.064 fm to study the scaling violation effect. We find only mild scaling violation when the lattice scale is determined by the nucleon mass. We obtain in the continuum limit, |α(NDR, 2 GeV)| = 0.0090(09)(+5 - 19) GeV3 and |β(NDR, 2 GeV)| = 0.0096(09)(+6 - 20) GeV3 with α and β in a relatively opposite sign, where the first error is statistical and the second is due to the uncertainty in the determination of the physical scale.