1. A similar case searching apparatus which searches a case database for a similar case data item similar to a target case data item of a target case to be diagnosed, the case database storing a plurality of case data items, each of the case data items and the target case data item including one or more medical images and an image interpretation report that is a document data item indicating a result of interpreting the one or more medical images, the similar case searching apparatus comprising: an image feature quantity extracting unit configured to extract a plurality of image feature quantities from an interpretation target image which is a medical image and obtained by carrying out a first test on a subject; a report analyzing unit configured to analyze a target image interpretation report which is generated by a user in interpretation of the interpretation target image, and divide descriptions in the target image interpretation report into a description related to the first test and a description related to a second test different from the first test carried out on the subject of the first test; a reference expression extracting unit configured to extract, from the description related to the second test divided by the report analyzing unit, one or more reference expressions each of which is a character string indicating a feature of a medical image, each of the reference expressions indicating a feature of a reference image obtained by carrying out the second test on the subject of the first test; a weight determining unit configured to determine, for each of the image feature quantities extracted from the interpretation target image by the image feature quantity extracting unit, a weight based on two-data correlation information that is prepared information defining a correlation between each of image feature quantities extracted from one or more medical images and each of one or more reference expressions extracted from an image interpretation report of the one or more medical images, the weight to the extracted image feature quantity having a value that is larger as the correlation between the image feature quantity and the reference expression is higher; and a similar case searching unit configured to search the case database for the similar case data item including a similar image similar to the interpretation target image, by weighting each of the image feature quantities in a first set extracted from the interpretation target image by the image feature quantity extracting unit and a corresponding one of the image feature quantities in a second set extracted from the one or more medical images included in the case data item registered in the case database, using the weight to each of the image feature quantities determined by the weight determining unit, and comparing the weighted image feature quantities in the first set and the weighted image feature quantities in the second set.