The invention is directed to largely improve reliability by surely protecting data on the basis of an emergency stop request even during a data transfer process. The invention provides a data memory system taking the form of a memory card or the like. When an emergency stop signal requesting an emergency stop is received from an information processor of a host during a read/write data transfer process, a control circuit immediately stops the transfer process and notifies the information processor of end of the read data transfer. At this time, the end of read data transfer is notified irrespective of whether the transfer is finished normally or abnormally. Even when a read data transfer request is received again from the information processor after notifying the information processor of the end of read data transfer, without transferring data, a controller notifies the information processor of an untransferable state of read data.