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, and 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 interpretations of the medical images, said similar case searching apparatus comprising: an image feature quantity extracting unit configured to extract a plurality of image feature quantities from one of the medical images which is an interpretation target image; a report analyzing unit configured to extract one of at least one image interpretation item and a disease name from the image interpretation report which is a target image interpretation report made by a user in the interpretation of the interpretation target image, the image interpretation item being a character string indicating a feature of a medical image and the disease name being obtained as a result of a diagnosis made by a user based on the medical image; a weight determining unit configured to determine a weight to each of the image feature quantities extracted by said image feature quantity extracting unit, based on two-data correlation information that is prepared information defining a correlation between each of the image feature quantities extracted from the medical image and one of the at least one image interpretation item and the disease name extracted from the image interpretation report of the medical image, such that the weight to the extracted image feature quantity is larger as the correlation between the image feature quantity and the one of the at least one image interpretation item and the disease name 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 said image feature quantity extracting unit and a corresponding one of the image feature quantities in a second set extracted from the medical images included in the case data items registered in the case database, using the weight to the image feature quantity determined by said weight determining unit, and comparing the weighted image feature quantities in the first set and the weighted image feature quantities in the second set, wherein when said report analyzing unit extracts, for a current one of the image feature quantities extracted by said image feature quantity extracting unit, both of the at least one image interpretation item and the disease name from the target image interpretation report, said weight determining unit is configured to determine, as the weight to the current image feature quantity, a product of a value indicating a correlation between the current image feature quantity and the at least one image interpretation item extracted by said report analyzing unit and a value indicating a correlation between the at least one image interpretation item and the disease name extracted by said report analyzing unit, based on the two-data correlation information for the current image feature quantity.