According to one embodiment, a command processing device includes a frontend part and a backend part. The frontend part is configured to execute a data communication with respect to a host based on a predetermined communication protocol, and accept a request of an execution of first and second commands from the backend part. The backend part is configured to queue commands including the first and second commands, and execute a data communication according to the first command with respect to an attachment device and a data communication according to the second command with respect to the attachment device in parallel.