In a storage device applying PCIe to a back-end network connection, in order to be capable of allocating bus numbers and making a PCIe switch expanded afterwards usable, it is necessary to once reset all PCIe switches. To dissolve this necessity, PCIe switches of the back-end network of the storage device are connected in series, a range of continuous bus numbers that are managed and stored in bus number management table is allocated for the back-end network connection, and when expanding the PCIe switch, the bus numbers are allocated in ascending order from a minimum value of the allocatable bus numbers to each of a link between the PCIe switches and to a virtual PCI bus within the PCIe switch, and the bus numbers are allocated in descending order from a maximum value of the allocatable bus numbers to the link between the PCIe switch and a drive.