Electrochemical switching of electrically conducting polymers from the conducting to the insulating state occurs very rapidly at first, followed by a slow relaxation with a logarithmic time dependence continuing for a few hours. The slow relaxation was simulated in a two-dimensional square lattice model by a Monte Carlo technique in the context of electrical percolation. A key aspect of the simulation is to pay attention to statistically aggregated conducting clusters. some of which are in electrical contact with the electrode. These clusters can participate in the conducting-to-insulating conversion. because they are at equi-potential to the electrode and work as a metal-like electrode. The insulating sites thus converted cut off the electrical path to the electrode and divide the cluster into small groups, most of which have lost electrical connection to the electrode. Clusters are rearranged by diffusion or electron exchange to yield new clusters and new electrical paths to the electrode. These processes were iterated by computation until all the conducting species were exhaustively converted. When the reaction rate of the conversion of the cathodic electrolysis was a rate-determining step, the amount of the simulated conducting species was linearly related to the logarithm of the electrolysis time. The simulated result was in agreement with the previous experimental observations with polyaniline films.