  We consider proton stability based on E_6 inspired extra U(1) model with S_4
flavor symmetry. In this model, a long life time of proton is realized by the
flavor symmetry. One of the interesting effects of flavor symmetry is that the
proton decay widths of p -> mu^+, X are suppressed by cancellation. This
suppression mechanism is important in the case that Yukawa coupling constants
are hierarchical. Our model predicts p -> e^+, K^0 has larger decay width than
that of p -> mu^+, K^0.
