1. An image registration device comprising: an image obtaining unit configured to obtain an intraoperative live view comprising two or more intraoperative images taken at different times, the intraoperative live view capturing a surgical site in a human body undergoing a surgery; an associated image obtaining unit configured to obtain an associated image associated with the surgery of the surgical site; and a registration unit configured to perform registration between the intraoperative live view and the associated image, wherein the registration unit extracts a plurality of feature points corresponding to one another from a registered intraoperative image having been registered with the associated image and a newly obtained intraoperative image, sets priority levels on the feature points corresponding to one another based on the associated image, obtains positional information indicating a relative positional difference between the registered intraoperative image and the newly obtained intraoperative image based on the feature points with the priority levels set thereon, and performs registration between the associated image and the newly obtained intraoperative image based on the positional information.