The first scan effect, which makes the voltammetric anodic peak of conducting polymers sharp and shifted in the positive direction only on the first scan, was investigated by determining the time-dependent concentration distribution of conducting species of polypyrrole films by means of the UV-visible diode array system with spatial resolution of 0.36 mm, The film in the insulating state was set on a glass plate and connected to an electrode at the end of the film. This experimental set-up corresponds to magnifying the film thickness by three or four orders of magnitude. On the first positive scan, a steep part of the concentration profiles was propagated from the electrode towards the top of the film, exhibiting a clear phase boundary. The anodic peak of the voltammogram appeared at the time when the conducting front reached the top of the film. In contrast, the conversion during the negative scan proceeded uniformly over the film and stopped at a conversion ratio of 0.9, which was regarded as a threshold value of the electric percolation. The second positive scan made the profile smooth. The dependence of the profile on the scanning direction agreed with the voltammetric features. The faradaic component of the voltammogram was constructed from the profile, and compared with the ordinary voltammogram.