Provided are a similar case search apparatus, a method for operating the similar case search apparatus, a program for operating the similar case search apparatus, and a similar case search system that can obtain a similar case desired by a doctor according to the purpose.A weighting processing unit (65) performs a weighting process for an individual lesion similarity (LS) which is a similarity between a target lesion and a case lesion of the same type, using a first weighting coefficient (CF1) corresponding to the degree of contribution of each type of lesion to the specification of a disease name, to calculate a processed individual lesion similarity (LSA-1) corresponding to a first purpose of specifying the disease name. In addition, the weighting processing unit performs the weighting process for the individual lesion similarity (LS), using a second weighting coefficient (CF2) corresponding to the degree of contribution of each type of lesion to the specification of a severity, to calculate a processed individual lesion similarity (LSA-2) corresponding to a second purpose of specifying the severity. A total similarity (TS) between a target image (20T) and a case image (20C) is calculated for each purpose on the basis of the processed individual lesion similarities (LSA-1) and (LSA-2).
