A failure diagnosis system includes a first database, a second database, an observation-information acquiring unit, a determination-probability calculating unit and an occurrence-probability calculating unit. A first cause-and-effect network stored in the first database stochastically represents a relationship between failure-type candidates and first observation information, which have cause-and-effect relationships with the failure-type candidates. Each second cause-and-effect network stored in the second database stochastically represents relationships between second observation information and failure-cause candidates. The observation-information acquiring unit acquires the first and second observation information from a diagnosed system. The determination-probability calculating unit calculates a determination probability of each failure-type candidate based on the first observation information and the first cause-and-effect network. The occurrence-probability calculating unit calculates occurrence probabilities of the failure-cause candidates for each failure-type candidate based on the second observation information and the second cause-and-effect networks.