A console of an X-ray imaging system functions as a query receiver and a retrieval section to support decision on an exposure condition. The query receiver receives a retrieval query such as an exposed body portion and an exposure direction. The retrieval section refers to an exposure date of image files having the same patient ID number, and calculates an exposure interval between a pair of prior and subsequent image files the exposure dates of which are the nearest to each other. The exposure interval is compared with a threshold value. If the exposure interval is less than the threshold value, neither image file is assigned as a model image file. If the exposure interval is the threshold value or more, the subsequent image file is assigned as the model image file. The retrieval section retrieves the image file matching the retrieval query out of the model image files.