1. An image management system for performing subtraction between two images of the same patient, comprising: an input unit that inputs a first image which is one of a processed image type for displaying the first image on a monitor and an unprocessed original image type; a registration decision unit that decides whether or not a second image having image identification information that is the same as image identification information of the first image is registered in the image management system, wherein if the first image is the processed image type, the registration decision unit decides whether a second image that is the unprocessed original image type and has image identification information that is the same as image identification information of the first image is registered, and if the first image is the unprocessed original image type, the registration decision unit decides whether a second image that is the processed image type and has image identification information that is the same as image identification information of the first image is registered; a first registration unit that creates a new entry in a database for the first image and registers the image identification information of the first image in association with the first image in a case where the second image is not registered in the image management system, and stores the first image in a storage unit; and a second registration unit that, in a case where the second image is registered in the image management system in an existing entry in the database and the image identification information of the second image is registered in the existing entry, updates in the existing entry for the second image, information indicating that the input first image is now registered, and stores the first image in the storage unit.