A similarity calculation apparatus includes: a relationship-information storage unit that stores relationships between configuration items belonging to an information system and stored in multiple databases; a first counting unit that compares, across the databases, attribute values of one configuration item in each of the databases, thereby counting a matching-attribute-value count; a second counting unit that identifies a configuration item having a relationship with the one configuration item in each of the database from the relationship-information storage unit and compares, across the databases, attribute values of the identified configuration items to count a matching-attribute-value count; and a similarity calculation unit that calculates similarity between the configuration items, of which matching-attribute-value counts each have been counted by the first counting unit or the second counting unit, based on the matching-attribute-value count counted by the first counting unit and the matching-attribute-value count counted by the second counting unit.