1. A medical image retrieval system comprising: a processor and memory; an image database which stores medical images; an interpretation unit which acquires a medical image for use in performing interpretation by a radiologist from the image database and provides the medical image to a computer terminal, wherein each of the medical images is associated with a report including numerical information about diagnosis and attribute information about the report; a similar example requesting unit which issues an image request requesting a reference image for use with the medical image in performing interpretation; an image retrieval unit which retrieves the reference image from the image database in accordance with the image request and provides the reference image to the computer terminal in order to propose the reference image as a reference for diagnosis to an user of the computer terminal; an evaluation input unit which prompts the user of the computer terminal to input an evaluation indicating whether the reference image has been helpful for diagnosis based on the currently diagnosed image, wherein the image retrieval unit calculates, if a condition provided by the radiologist is satisfied with respect to the attribute information, a similarity degree between a first vector including numerical information of a first report associated with the medical image and a second vector including numerical information of a second report associated with a candidate reference image, and determines the candidate reference image associated with the second report as the reference image in accordance with the calculated similarity degree; and wherein the attribute information includes at least one of the number of links extending from another report to the candidate reference image associated with the second report, the number of times the candidate reference image associated with the second report has been accessed, an evaluation value of reliability of a radiologist identified in the second report, and the number of times of appearance of a specific word in the second report.