In a plural computer system executing a virtual computer, an exterior storage volume may receive unjustly multiple access, and contents of the volume may be destroyed. Provided are: a switch coupling a virtual computer and I/O, a virtual computer managing unit coupled to a computer and the switch, and a determination unit determining a login acceptance/rejection of a virtual computer and I/O. The virtual computer possesses a virtual HBA, and upon receiving a login inquiry to the I/O from a certain virtual computer, a determination unit compares an identifier allocated to the certain virtual computer with an identifier allocated to another virtual computer to be executed on a physical computer different from the physical computer on which the certain virtual computer is executed, and determines login acceptance/rejection. Accordingly, an access control to the I/O is performed.