Tantalum solid electrolyte capacitor is in practical use due to its excellent features such as a small mass and large capacitance. Since the resources of tantalum are rare, they are expensive and price fluctuates by the unstable supply. Recently, many attentions have been made for niobium as an alternative anode material. The relative dielectric constant of a niobium anodic oxide film is about 40, and this enables us for high capacitance. However, the leakage current of niobium anodic oxide film is higher than that for tantalum, and this is strongly affected by the heat treatment in the fabrication process and the bias potential. To overcome this problem, many researches have done, but they were focused on the oxide film | electrolyte solution interface. In this research, we focused on the effect of the water impurity in organic electrolyte on the leakage current at an oxide film | solid electrolyte interface of which was similar to an actual solid electrolyte capacitor.