1. An image processing device comprising: correspondence relationship setting means for setting a three-dimensional common coordinate system and for setting a first correspondence relationship between each pixel of a first three-dimensional image which has at least a portion of a human body as a subject and coordinates on the common coordinate system; first correspondence relationship acquisition means for aligning a second three-dimensional image, which has at least a portion of the human body as a subject that at least partially overlaps the subject in the first three-dimensional image, with the first three-dimensional image to calculate a correspondence relationship between pixels of the first three-dimensional image and the second three-dimensional image, and for calculating a second correspondence relationship between each pixel of the second three-dimensional image and coordinates on the common coordinate system, on the basis of the calculated correspondence relationship and the set first correspondence relationship; storage means for storing the first correspondence relationship and the second correspondence relationship; obtaining means for obtaining the first correspondence relationship and the second correspondence relationship; and second correspondence relationship acquisition means for obtaining a correspondence relationship between the first three-dimensional image and the second three-dimensional image based on the first correspondence relationship and the second correspondence relationship; wherein, when a subject in a new three-dimensional image whose correspondence relationship with coordinates on the common coordinate system has been stored in the storage means as least partially overlaps the subject in the existing three-dimensional image whose correspondence relationship with coordinates on the auxiliary coordinate system has been stored in the storage means, the correspondence relationship acquisition means aligns the existing three-dimensional image with the new three-dimensional image to calculate a correspondence relationship between pixels of the existing three-dimensional image and the new three-dimensional image, and calculates another correspondence relationship between each pixel of the existing three-dimensional image and coordinates on the common coordinate system, on the basis of the calculated correspondence relationship and the correspondence relationship between the new three-dimensional image and the coordinates on the common coordinate system stored in the storage means, and the storage means stores the calculated another correspondence relationship, instead of the stored correspondence relationship between the existing three-dimensional image and the coordinates on the auxiliary coordinate system.