1. A similar case searching apparatus which searches a case database for similar case data items including similar images similar to an interpretation target image, the case database storing a plurality of case data items, and each of the plurality of case data items and a case data item of the interpretation target image 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 that is the interpretation target image; an image interpretation item fitting degree calculating unit configured to calculate a fitting degree of the image feature quantities extracted by said image feature quantity extracting unit with respect to each of image interpretation items, based on first image interpretation knowledge composed of information items each indicating a presence range of values of image feature quantities of a corresponding type extracted from medical images whose image interpretation reports include the image interpretation item, each of the image interpretation items being a character string verbally indicating a feature of the medical images, the information items being prepared for the respective image interpretation items, and the values being calculated with respect to each of the image interpretation items; an image interpretation item candidate display unit configured to display one of (a) image interpretation items each having a fitting degree larger than a predetermined threshold value and (b) a predetermined number of image interpretation items selected in a descending order of their fitting degrees, both (a) and (b) being included in the image interpretation items; an image interpretation item selecting unit configured to allow a user to select one or more of the image interpretation items displayed by said image interpretation item candidate display unit; a weight determining unit configured to determine, based on second image interpretation knowledge, a weight that is added to each of the image feature quantities extracted by said image feature extracting unit, such that the weight to the image feature quantity is larger as a correlation between the image feature quantity and the one of the image interpretation items selected by said image interpretation item selecting unit is higher, the second image interpretation knowledge being prepared information defining correlations between image feature quantities extracted from medical images and image interpretation items selected from the image interpretation reports of the medical images; and a similar case searching unit configured to search the case database for the similar case data items, 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.