Standby computers are dynamically divided into groups according to the difference between the software thereof and the software required of a business system. When a computer is made available, the standby computers divided into groups by software structure are searched and an appropriate one is extracted to quickly complete the construction of the software environment. An active computer, if to be transferred to standby mode, is associated with the group having the same software structure as the business system with which the active computer has thus far operated. The active/standby states of the computers are monitored, and the standby computers are changed thereby to widen the possible range of application to other business systems, removing copy operation of disk images from the active computer to the standby computers.