Even if the number of comparison target patients' cases is small, weighting is appropriately performed based on clinical-information items, and a degree of similarity between a target patient's case and a case of a comparison target patient is calculated. Likelihood ratio between a likelihood of belonging to one classification of a key item and a likelihood of belonging to other classification of the key item when a case belongs to each classification of a clinical-information item other than the key item is calculated, based on registration case information for calculating a likelihood ratio, for each classification of a key item. A weighting coefficient corresponding to each classification of the clinical-information item other than the key item for each classification of the key item is determined based on a target classification of a target clinical-information item and the calculated likelihood ratio. A degree of similarity is calculated for each registration case included in registration case information for calculating a degree of similarity by using weighting information corresponding to each classification of the key item and each classification of the clinical-information item other than the key item.
