A semiconductor memory device includes a memory which comprises a confidential information area storing confidential information and a flag. A controller reads the flag from the memory when instructed to erase or write data in the confidential information area, determines whether the flag is set, erases or writes data in the confidential information area when the flag is clear, and abandons a process requested by an erase or write instruction when the flag is set. An authenticator uses data in the confidential information area to execute an operation for authentication. A management information area may store management information for associated pages. The flag may include a bit string and a complementary bit string to improve reliability of the flag. The confidential information area may store dummy data when the memory is used for uses other than an application with an authentication function, so no problem arises using a normal controller.