A redundant computing system is composed of two systems: a first arithmetic processing unit (A-system) and a second arithmetic processing unit (B-system) having the same functions. A diagnosis control unit performs diagnosis of one system while the other system is performing arithmetic processing operation. The diagnosis control unit controls the input to the first and second arithmetic processing units by way of an input control unit according to the diagnosis operation, and an output control unit controls the output from the first and second arithmetic processing units according to the diagnosis result. After termination of the diagnosis, a value is copied from a storage unit of the system which has not been diagnosed to a storage unit of the system which has been diagnosed, and the redundant computing system resumes the redundant operation.