PURPOSE: To obtain a redundant system having high reliability without duplexing totally the system by providing the processors into the system as the redundant components together with a cooling device, etc.

CONSTITUTION: If one of processors(PRC0 - PRCn) 110 - 11n has a fault in a system operating state, a system unit controller SUC 5 applies an interruption to a host OS to separate the faulty processor from other nondefective processors. Then the SUC 5 cuts off the supply of power to the faulty processor and separates this processor from a system. At the same time, the SUC 5 orders the optional one of nondefective processors PRC0 - n to load a microprogram via a microcode storage MSC 7 as long as the system operation can be carried on with reduction of the system operating scale. So that a spare processor (PRCn + 1) 11n + 1 or the optional nondefective processor is started. In such a constitution, the reliability of a redundant system is improved.