1. A first computer in a virtual computer system comprising a plurality of computers for executing a program and a storage system connected to the computers via a network, the first computer comprising: a virtualization mechanism which generates a first virtual computer and a second virtual computer to each of which a physical CPU and a physical memory of the first computer are divided and assigned, the first virtual computer and the second virtual computer accessing a plurality of storage volumes on the storage system, wherein a first management table is stored in a memory of the virtualization mechanism, the management table indicates a first correspondence between the first virtual computer and whether activation of the first virtual computer is permitted or prohibited and a second correspondence between the second virtual computer and whether activation of the second virtual computer is permitted or prohibited, where the first correspondence and the second correspondence are updated independently of each other, wherein a second management table is stored in the memory of the virtualization mechanism, the second management table indicates a third correspondence between an identifier and a logic I/O adapter assigned to the first virtual computer, wherein when a request for activating the first virtual computer is received, the virtualization mechanism in the first computer executes a first judgment in which: the virtualization mechanism judges whether or not the activation of the first virtual computer is prohibited based on the first management table and if, in accordance with the first judgment, the activation is not prohibited, the virtualization mechanism executes a second judgment of checking the identifier corresponding to the logic I/O adapter assigned to the first virtual computer based on the second management table when the first virtual computer is to be activated, and prohibits the activation of the first virtual computer when the identifier is registered as an invalid identifier, and if, in accordance with the second judgment, the activation is not prohibited, activates the first virtual computer.