What is claimed is:1. An adapter configured to be inserted into a slot of an electronic device and also configured to connect each of a Peripheral Component Interconnect (PCI) board and a Peripheral Component Interconnect-express (PCIe) board to a control board that is installed on a side of the slot toward which the adaptor is inserted, the adapter comprising a base plate,wherein the base plate comprises:a holding structure configured to selectively hold a Peripheral Component Interconnect (PCI) relay board configured to connect the PCI board to the control board and a Peripheral Component Interconnect-express (PCIe) relay board configured to connect the PCIe board to the control board; andan attachment structure configured to selectively attach a Peripheral Component Interconnect (PCI) bracket configured to fix the PCI board to the base plate and a Peripheral Component Interconnect-express (PCIe) bracket configured to fix the PCIe board to the base plate,wherein the attachment structure includes a plurality of insertion holes,the plurality of insertion holes includesone or more first insertion holes that allows a fastening member to be inserted into the base plate wherein the fastening member attaches the PCI bracket to the base plate, andone or more second insertion holes that differs from the one or more first insertion holes and allows a fastening member to be inserted into the base plate wherein the fastening member attaches the PCIe bracket to the base plate.2. The adapter according to claim 1, wherein the holding structure includes:a groove into which each of the PCI relay board and the PCIe relay board is fitted; anda retainer configured to hold each of the PCI relay board and the PCIe relay board fitted into the groove.3. The adapter according to claim 2, wherein the retainer includes an engaging part configured to engage with a notch or an engagement hole formed in each of the PCI relay board and the PCIe relay board.4. The adapter according to claim 1, wherein the attachment structure includes one or more insertion holes, and a fastening member for attaching the PCI bracket or the PCIe bracket to the base plate is inserted into the base plate through each of the insertion holes.5. The adapter according to claim 4, wherein the fastening member is inserted into at least one of the one or more insertion holes in both cases where the PCI bracket is attached to the base plate and where the PCIe bracket is attached to the base plate.6. The adapter according to claim 1, wherein:the PCI relay board includes:a first connector configured to be connectable to the control board; anda second connector configured to be connectable to the PCI board; andthe PCIe relay board includes:the first connector; anda third connector configured to be connectable to the PCIe board.7. The adapter according to claim 1, wherein the base plate is provided with the PCI relay board and the PCI bracket.8. The adapter according to claim 1, wherein the base plate is provided with the PCIe relay board and the PCIe bracket.