A next-generation OS with a virtualization feature is executed as a user program on a first virtual processor by selecting, in response to a cause of a call for a host VMM, one of a guest status area (221) for executing a user program on a second virtual processor and a host status area (222) for executing the guest VMM, and by updating a guest status area (131) of a shadow VMCS for controlling a physical processor. Accordingly, without a decrease in performance of a virtual computer, the next-generation OS incorporating the virtualization feature is executed on a virtual server, and the next-generation OS and an existing OS are integrated on a single physical computer.