At least one of components of a multiplex system can detect a failure by itself. An output of the multiplex system is determined from outputs of the components and a failure detection notification. Thus, a multiplex system having higher reliability is provided. A multiplex system 1 includes a plurality of components including a component A1 (10a), a component B1 (10b), and a component C1 (10c) that cannot detect a failure by itself and a component A2 (20a) and a component B2 (20b) that can detect a failure by itself. Outputs 11a, 11b, 11c, 21a, and 21b of the components and failure detection notifications 22a and 22b are inputted to an output determination part 30. The output determination part 30 determines an output of the multiplex system 1 from the outputs of the components and the failure detection notifications.