A high reliability system in which a plurality of apparatuses each having a variable function unit and a fixed function unit are mutually connected through an internal network. Pairs of apparatuses are constructed and spare apparatuses are allocated, thereby providing redundancy. Each apparatus has a service processing unit which executes the software for providing a peculiar service to every apparatus pair, a synchronization processing unit which synchronizes the services between the apparatus pair, a fault detecting unit which detects a fault of the pair partner apparatus, and a fault processing unit which, issues a pair request to the network and reconstructs an apparatus pair together with the apparatus to which the pair application has been made. The redundancy recovery after the exchange of the failed apparatus is executed automatically, thereby shortening a time in a non-redundant state.