Failure detection device for an external sensor and a failure detection method for an external sensor

A failure detection device that detects a failure in a plurality of external sensors onboard a vehicle, the failure detection device comprises: an overlapping region storage unit that stores an overlapping region of detection areas of the external sensors; an environment performance storage unit that stores an environment-dependent performance of sensing of the external sensors; an environment information acquisition unit that acquires environment information about the vehicle; a recognition result comparison unit that compares recognition results of an object in the overlapping region from the external sensors; a failure likelihood computation unit that computes a failure likelihood of each external sensor based on a comparison result of the recognition results, the environment information, and the environment-dependent performance; and a failure determination unit that determines a failure in each external sensor based on the failure likelihood of each of the external sensors.