We have investigated leakage conduction transport in an amorphous resistive-switching NbOx thin film sandwiched between Pt electrodes. The film was grown using a pulsed laser deposition technique, and showed unipolar-type resistance switching characteristics. The current-voltage characteristics in the initial insulating resistance state (IS) and in the bistable high-resistance state (HRS) were modeled by using various leakage conduction mechanisms. The electroforming forming process causes dominant conduction mechanisms in the IS and the HRS to differ. Thermionic emission is observed both in the IS and the HRS, but the thermal activation energy in the HRS is smaller.