1. An operating method of a storage device including a nonvolatile memory and a memory controller configured to control the nonvolatile memory, the operating method comprising: 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; and inhibiting, when a full completion queue exist, the selecting from a full submission queue corresponding to the full completion queue among the plurality of submission queues until the full completion queue becomes a normal completion queue, wherein the plurality of submission queues are sequentially selected in accordance with a round robin scheduling.