Electrically nonconductive "white" polyaniline film, which was reduced electrochemically with hydrazine, was exposed to air to be oxidized and gradually became violet. The degree of the coloration increased linearly with the logarithm of the exposure time. The time dependence was found to obey slow relaxation based on the concept of percolation theory. The oxidation is promoted by the visible light irradiation with about 600 nm, which corresponded to the transition between the valence band and the antibonding bipolaron band of polyaniline. From these observations, we predict that the oxidation by oxygen is associated with surface reactions like electrochemical processes and that it is accelerated by holes generated with the irradiation.