For the purpose of clarifying the structure of anodic oxide film of tantalum, photo-effects and variation of capacity with applied bias potential were studied for relatively thin anodic oxide films. From the results it is concluded that a p-i-n junction is formed in the oxide film. The construction of junction is as follows. In contact with the base metal there are very thin n-layers, and on the layers there lie nearly stoichiometric i-layers. The surface p-layers lying above i-layers are also very thin. The thickness of the i-layers is proportional to the anodization voltage, and the thickness of the extrinsic layers is nearly independent of the anodization voltage and may be in the range of 20–50 Å. The donor levels consist of excess tantalum atoms and the acceptor levels consist of excess oxygen atoms (or defects of tantalum atoms) as well as adsorbed oxygen on the oxide surface. Due to nearly amorphous structure of the oxide, the recombination rate of injected carriers is high in the junction. Coincidence between the theory of p-i-n junction and the experimental results is quite good.