A computer system includes: a first HBA that constitutes an operational path and on which a first PCI/PCIe-bus expansion card having a PCI PM function, which is a power saving function, is mounted; and a second HBA that constitutes a standby path and on which a second PCI/PCIe-bus expansion card having the PCI PM function is mounted; and a control section that controls the first HBA and the second HBA. The control section first uses the operational path by setting the first PCI/PCIe-bus expansion card to an active state. When a failure occurs in the operational path, the control section performs control such that communication recovery processing for the operational path is applied to the first HBA and further the second PCI/PCIe-bus expansion card of the second HBA is changed from a power saving state to which the second PCI/PCIe-bus expansion card has been first set, to an active state. When the communication recovery processing ends unsuccessfully, the control section performs communication with the I/O device by using the second PCI/PCIe-bus expansion card of the second HBA.