A computer system has a plurality of computer nodes, and each computer node has a plurality of virtual computers and a control base unit controlling the virtual computers. Each virtual computer constitutes a multiplexing group with another virtual computer operating on another computer node different from its own computer node, with one operating as the master and the other as the slave. The control base unit controls whether each virtual computer is operating as either the master or the slave, and monitors the respective states of each virtual computer. The control base unit, when it has detected in its own node a failure of the virtual computer operating as the master virtual computer, makes a decision whether to also switch the other virtual computers operating on its own computer node from master virtual computers to slave virtual computers with the virtual computer in which the failure occurred.