PURPOSE: To relieve a load on an active system processor and to improve the processing efficiency by allowing a standby system processor in the standby state in the normal operation to take over part of the processing of an active system processor when the active system processor is overloaded.

CONSTITUTION: When an active system processor 11 is overloaded, part of the processing is executed by a standby system processor 12 to relieve the load of the processor 11 and to allow the system to execute the processing thereby executing the processing without call restriction or the like to the utmost even when calls take place simultaneously. Moreover, a flag is set to a service control table and the service added newly is executed by the processor 12, and should there be a bug or the like in the service program and destruction of data be caused, the effect is limited in the standby system including the processor 12 and not on the active system including the processor 11. Thus, no adverse effect is exerted onto an existing service and the reliability of the system is improved.