According to one embodiment, a controller receives from a host a first command for writing data sequentially to a first zone. When buffers include a first buffer to which the first zone is allocated, the controller stores the first data in the first buffer. When the buffers do not include the first buffer but include a free buffer, the controller allocates the first zone to a second buffer that is the free buffer and stores the first data in the second buffer. When the buffers include neither the first buffer nor a free buffer, the controller additionally allocates the first zone to a third buffer of which last update for storing data is the oldest among the buffers, and stores the first data in the third buffer to which the first zone is additionally allocated.