PURPOSE: To improve the operating efficiency of a virtual computer by reserving this computer to a real input/output device which uses the virtual computer before its operation.

CONSTITUTION: The real and virtual input/output devices are controlled by the real and virtual input/output device tables respectively. A command is inputted to set the real input/output device used by the virtual computers 24 and 25 in a virtual computer reservation state before the computers 24 and 25 are started. Thus a virtual computer monitor 1 inputs the name of the real input/ output device and searches the corresponding entry of the real input/output device table. If this entry is kept free, the virtual computer reservation state is set and real input/output device can be allocated and used by the virtual computers. Thus it is possible to evade a waiting state of the virtual computer for the real input/output device which is caused by the conflict to a job of the real computer. Then the virtual computers are smoothly operated.