Embodiments of the invention ensure both a realtime nature of a realtime processing and data integrity of a non-realtime processing and perform the both processings efficiently when the realtime processing and the non-realtime processing are performed simultaneously. In one embodiment, a time limit is set not only for a realtime processing command but also for a non-realtime processing command and, if the execution of the non-realtime processing command is not completed within the set time limit, the execution of the non-realtime processing command is interrupted forcibly and a host is informed of a data transfer status at the time of the interruption so that the host can restart the interrupted processing based on the data transfer status of the interrupted processing when a time that can be allocated for the non-realtime processing command occurs again.