An image acquisition unit acquires an actual endoscopic image that is generated by an endoscope inserted into a tubular structure of a subject and represents an inner wall of the tubular structure, and a virtual endoscopic image that is generated from a three-dimensional image including the tubular structure of the subject and spuriously represents the inner wall of the tubular structure. A conversion unit converts an expression form of the actual endoscopic image into an expression form of the virtual endoscopic image. A similarity calculation unit calculates similarity between the converted actual endoscopic image and the virtual endoscopic image.