An image acquisition unit sequentially acquires an endoscope image of a tubular structure having a plurality of branch structures, and an image generation unit generates an image of the tubular structure. A first certainty factor calculation unit calculates a first certainty factor indicating a possibility of presence of the endoscope within the tubular structure. A second certainty factor calculation unit calculates a second certainty factor indicating a possibility of presence of the endoscope by performing matching between the image of the tubular structure and each of the endoscope images at each of a plurality of positions within the tubular structure. A current position specifying unit specifies the current position of the endoscope based on the first and second certainty factors.
