Disclosed herewith is a composite type computer system that can assure that a PCI tree to be allocated to a computer is configured completely before the computer is powered. The composite type computer system includes a PCI switch that connects plural computers through PCI interfaces; plural PCI devices connected to the PCI switch; a system controller that controls the computers; and a PCI manager that controls allocation of the PCI devices to the computers. The system controller carries out processings in the steps of (a) powering an object computer to start up its OS; (b) acquiring the identifier of a PCI tree allocated by the system controller to the computer and PCI tree management information denoting the status of the PCI tree; (c) retrying the powering or canceling the powering of the computer if the acquired PCI tree management information denotes the status “not initialized”; and (d) carrying out the powering for the computer if the acquired PCI management information denotes the status “initialized”.