A bus-protocol converting device includes: a command detecting unit that detects a command sent from an external-memory control device, connected to a primary bus, to a primary bus interface controller; a command converting unit that converts the detected command into a command to be sent from a secondary bus interface controller to an external memory device through a secondary bus; a status detecting unit that detects a status sent from the external memory device; a status converting unit that converts the detected status into a status to be sent from the primary-bus interface controller to the external-memory control device through the primary bus; and a data transfer controller that is provided between the primary bus interface controller and the secondary bus interface controller to perform data transfer between the external-memory control device and the external memory device through a DMA bus.