PURPOSE: To effectively utilize the system resources by produce and add optionally the guest OSs desired by a virtual computer and at the same time deleting optionally the unnecessary guest OSs.

CONSTITUTION: A guest OS dynamic production means 5 produces and adds optionally the desired guest OSs, and a guest OS dynamic deletion means 6 deletes optionally the unnecessary guest OSs. A guest OS resources allocation means 8 allocates the necessary resources to the produced and added guest OSs, and a guest OS resources opening means 9 opens the resources allocated to the deleted guest OSs. Then a guest OS communication means 7 secures the synchronization between the guest OS1 and OS2 and a virtual computer VM monitor 4 for execution of their operations. Thus the environment of the virtual computer can be optionally changed between a single OS environment and a multi-OS environment in accordance with the requests of users. Then the resources can be effectively utilized.