1. A similar case searching apparatus which searches out, from a case database, at least one second case data item similar to a first case data item including a first medical image group of first medical images and a first image interpretation report which is a text data item indicating a result of interpreting the first medical image group, the similar case searching apparatus comprising: an image feature extracting unit configured to extract a plurality of image feature quantities from the first medical image group; a keyword extracting unit configured to extract a keyword from the first image interpretation report, the keyword being either (a) an image interpretation item which is a character string indicating a feature of at least one of the first medical images or (b) a disease name which is a result of a diagnosis made by a user based on the first medical images; a keyword attribute obtaining unit configured to obtain an attribute value which is a word indicating a supplemental concept of the keyword, from a sentence including the keyword extracted by the keyword extracting unit; a search vector generating unit configured to: with reference to a relevance database storing the degrees of relevance between (i) a combination of the keyword extracted by the keyword extracting unit and the attribute value of the keyword obtained by the keyword attribute obtaining unit and (ii) the respective image feature quantities extracted by the image feature extracting unit, perform weighting on (i) the image feature quantities extracted by the image feature extracting unit and (ii) image feature quantities extracted from a second medical image group of second medical images included in the at least one second case data item stored in the case database, using the degrees of relevance as weights; and generate a search vector for the first medical image group and a search vector for the second medical image group, each of the search vectors having, as elements, corresponding ones of image feature quantities resulting from the weighting; and a similar case searching unit configured to search out the at least one second case data item stored in the case database, based on a degree of similarity between the search vector for the first medical image group and the search vector for the second medical group.