Concentration distribution of conducting species of polypyrrole films in the electrochemical undoping process was determined by a spectroscopic method using a photodiode array in the simulated film experiment. A polypyrrole film in the doped state was set on an insulating glass plate and connected to an electrode at the end of the film. The length of the Nm corresponds to the film thickness in the ordinary experiment of film-coated electrodes. The film was reduced or undoped by the potential step. Absorbance responding to the potential step was monitored at 14 segments of the film by the photodiode array with effective spatial resolution of 0.36 mm. Then the concentration distribution of the oxidized conducting species was determined every 50 ms. The conversion occurred not only near the electrode but also at the top of the film. It stopped at the conversion ratio 0.87. 13% of the conducting species was left behind from the reduction in the uniform distribution. The incomplete conversion was attributed to electrical disconnection of conducting clusters from the electrode on the basis of the percolation theory.