An operating method of a storage device including a nonvolatile memory and a memory controller configured to control the nonvolatile memory may include selecting one of a plurality of submission queues; fetching, from the selected submission queue, a command for accessing the storage device; executing the fetched command; outputting an execution result of the fetched command to a selected completion queue corresponding to the selected submission queue; determining whether a submission queue is a full submission queue, the full submission queue corresponding to a full completion queue; and inhibiting the selecting from selecting the full submission queue until the full completion queue becomes a normal completion queue, when the determining determines a submission queue is the full submission queue, wherein the plurality of submission queues are sequentially selected in accordance with round robin scheduling.