Effects of nitrogen doping on the electric insulation of anodically oxidized films of Ta have been investigated. As to the undoped oxide films, the relation between electric leak current and applied voltage changed from ohmic to space charge limited and finally to Poole-Frenkel relation as the voltage was increased. The leak current increased by a factor of 10² at an applied voltage of 20V as a result of annealing at 350°C. On the other hand, the relation between electric leak current and voltage of nitrogen doped oxidized films was only ohmic, independent of the applied voltage and the leak current at 20V was 10⁵∼10⁷ times as small as that of undoped films. The decrement depended greatly on the amount of doped nitrogen, and annealing of films also caused the current to decrease further. These experimental facts suggest an important role of the nitrogen atoms in the insulation of the oxide films. These phenomena can be interpreted by that the number of electrons contributing to the electric conduction becomes small because Ta atoms which are not bonded to oxygen can be terminated by the doped nitrogen atoms.