Dual system

A dual system according to the present invention includes: a memory copying unit configured to, when an arithmetic device of a first computer module is installed into the dual system, execute a memory copy process of copying data in a memory region of a second computer module into a memory region of the first computer module; a substitute processing unit configured to execute a service substitute process that is executed by a different arithmetic device from an arithmetic device executing the memory copy process and that is part of processes involved in the information processing service by the dual system; and a shared memory that stores data of the service substitute process by the substitute processing unit. The shared memory is excluded from the target of the memory copy process.