PURPOSE: To switch a working computer having the breakdown to a spare computer with no intervention of a manual operation by setting a working/spare control mechanism having a communication means between the working and spare computers.

CONSTITUTION: A working side control part 16a and a spare side control part 16b of a computer 11 of a working system and a computer 12 of a spare system respectively transmit and receive messages with each other for each fixed time and monitor the time. In case the computer 11 has the abnormality and an answer is not acquired from the part 16a within a fixed time, a system switching instruction is produced to switch the computer 11 to the computer 12. Receiving said instruction, a decentralized control mechanism 12b informs a fact that the computer 11 of a site A is switched to the computer 12. At the same time, both the mechanism 12b and a data base/data communication control mechanism 12a work in the working system of the site A and therefore start the rise actions, i.e., the recovering actions.