According to one embodiment, a memory system includes a nonvolatile first memory, a nonvolatile second memory, a data-copy processing unit and a data invalidation processing unit. The first memory has a storage capacity for n (n≧2) pages per word line. The nonvolatile second memory temporarily stores user data write-requested from a host apparatus. The data-copy processing unit executes data copy processing including reading out, in page units, the user data stored in the second memory and sequentially writing the read-out user data in page units in the first memory. The data invalidation processing unit selects, after the execution of the data copy processing, based on whether the memory cell group per word line stores user data for n pages, user data requiring backup out of the user data subjected to the data copy processing and leaves the selected user data in the second memory as backup data.