PURPOSE: To assure the continuity of data immediately after a changeover by storing once current system data into an input output module for an inter- system link and thereafter, transferring them to a standby system when a changeover is executed from the current system and the standby system.

CONSTITUTION: A changeover request signal from a manual system changeover switch 8 generates an interruption through system changeover interrupting modules 2a and 2b to CPUs 6a and 6b. The CPUs 6a and 6b judge from changeover condition input devices 5a and 5b whether the connection of a switch 1 is the current system or the standby system. At the time of the current system, the data (receiving data, transmitting data, transmitting waiting address, etc.,) for inter-system link are outputted to an input module 3a for an inter-system link, and thereafter, a system changeover signal is outputted to system changeover output devices 4a and 4b and shifted from the current system to the standby system. At the time of the standby system, the data are received from the input output module 3b, set to a main memory device 7b and the shifting waiting from the standby system to the current system is obtained. Thus, the continuity of the data immediately after the changeover is secured.