1. An information processing unit that displays a diagnostic image serving as a diagnosis target and a diagnosed image similar to the diagnostic image for comparison, the information processing unit comprising: a diagnostic image input section that inputs the diagnostic image; an operation information obtaining section that obtains display operation history information representing an operation history of a user who controls displaying of the diagnostic image; a query image generation section that extracts a predetermined region of the input diagnostic image to generate a query image; a diagnosed image obtaining section that supplies the generated query image and the display operation history information to a diagnosed image search unit and obtains the diagnosed image obtained as a search result by the diagnosed image search unit; and a display control section that displays the diagnostic image and the obtained diagnosed image for comparison, wherein the diagnosed image search unit includes an image feature amount extraction section that extracts an image feature amount of the query image, a search section that retrieves diagnosed images each of which includes a sub-image with an image feature amount similar to the image feature amount of the query image from diagnosed images registered in advance, and a primary filter section that extracts a diagnosed image with display operation history information similar to the display operation history information corresponding to the diagnostic image that is a base of the query image from the diagnosed images retrieved by the search section.