A memory system includes a nonvolatile semiconductor memory which includes a first original block composed of n (n being natural number) write unit areas and a first subblock composed of a plurality of write unit areas. A controller writes data having one of first to p-th (p being natural number smaller than n) addresses into the first original block. The controller writes data which has a first write address of one of the first to p-th addresses into the first subblock when the controller receives request to write data having the first write address and data having the first write address exists in the first original block.