A virtualization mechanism in a first computer of the plurality of computers generates at least one virtual computer on the first computer. The virtualization mechanism in the first computer executes at least one of a first judgment and a second judgment when a request for activating a first virtual computer of the at least one virtual computer is received, the first judgment being for judging whether or not the activation of the first virtual computer is prohibited based on first information stored in a memory of the virtualization mechanism, and the second judgment being for judging whether or not an identifier included in a logic I/O adapter device assigned to the first virtual computer is invalid based on second information stored in the memory of the virtualization mechanism. The activation of the first virtual computer is permitted or prohibited based on the executed judgment.