1. A virtual computer system comprising: a real machine including first and second input/output devices; a plurality of virtual machines being operated on the real machine and including first and second virtual machines; an accepting and analyzing means for accepting and analyzing a single command which instructs to swap the first input/output device allocated to the first virtual machine with the second input/output device allocated to the second virtual machine; a detaching means for detaching the first and second input/output devices from the first and second virtual machines respectively in response to input of the command; and an attaching means for attaching to the first virtual machine the second input/output device detached from the second virtual machine and for attaching to the second virtual machine the first input/output device detached from the first virtual machine.