1. A storage device, comprising: a nonvolatile memory device comprising a plurality of memory cells, the memory cells divided into a plurality of pages; and a controller configured to control the nonvolatile memory device, wherein the storage device is configured to collect two or more write data groups to be written to two or more pages, respectively, to simultaneously perform a common write operation with the two or more pages, and then, to sequentially perform an individual write operation with respect to each of the two or more pages, wherein, in each of the two or more pages: memory cells connected to the same bit line and the same word line are programmed to a same program state in the common write operation and, program states of first memory cells connected to the same bit line and the same word line are maintained and second memory cells connected to the same bit line and the same word line are programmed to different program states in the individual write operations.