An image acquisition unit acquires a first medical image in a first expression form and a second medical image in a second expression form different from the first expression form. In a case where a conversion unit converts the first expression form of the first medical image and the second expression form of the second medical image into a third expression form, a converted first medical image and a converted second medical image are acquired. A similarity calculation unit calculates similarity between the converted first medical image and the converted second medical image.
