A combination identification unit (27) identifies combinations of one or more components which constitute a target system and in each of which an intrusion detection system that detects unauthorized access can be installed. A combination reduction unit (28) extracts, from the combinations identified by the combination identification unit, a combination that satisfies an installation condition accepted by an installation condition input unit (22) and can detect unauthorized communications indicated by attack information accepted by an attack information input unit (24) at a rate higher than or equal to a threshold.