The percolation threshold potential, at which electric connection of the conducting species generates a well-connected network, is a measure of electric conduction of the film under the electrochemical control of conducting polymers. Since the percolation belongs to critical phenomena, the conductivity should show a sharp change at this potential, as opposed to a smooth change of the Nernst relation. It was evaluated by a technique of a polyaniline film-bridged twin electrode. An alternating potential with a small amplitude superimposed on a constant potential was applied to one electrode. The direct component of the potential controlled the redox state of the film and the alternating component played a role in a signal for the conduction. The other electrode monitored the alternating amplitude, which was a measure of whether the film was percolated or not. The amplitude vs direct potential curve exhibited a sharp variation at 0.195 V vs see when the direct potential was swept positively. The sharply variational point was independent of the potential sweep rate, film thickness, the distance between the two electrodes, the applied potential amplitudes and frequencies. Thus it was regarded as the threshold potential. It was more negative than the growth potential for the conducting zone by 25 mV, which was the overpotential of the dynamical growth. When the potential scan was reversed, the sharply variational points shifted negatively by 0.20-0.25 V, and depended on the sweep rate, the film thickness and the distance between the electrodes. The time-dependence obeyed the rule of the logarithmical slow relaxation. By the technique of absorption of a beam with spatial resolution, the distribution of the conducting species was slightly non-uniform at potentials less than the threshold value.