According to an embodiment, a controller is connected to an external storage device and controls access to a semiconductor storage device including blocks each including memory cell groups each having memory cells. The block includes pages associated with each memory cell group. A writing process for each memory cell group includes writing stages. The controller includes a determining unit configured to determine data to be transferred to the page required in the writing process for a first memory cell group before the writing stage first starts when the writing stage is performed; a reading unit configured to read the determined data from the semiconductor storage device and to store the read data in the external storage device before the writing stage starts; and a writing unit configured to perform the writing process using the data stored in the external storage device when the writing stage is performed.