To provide a safer method with which an encryption key can not be extracted even when information in a storage circuit is read, since there is a defect that the encryption key can be extracted by reading held contents in the storage circuit although a mobile wireless telephone device stores in the internal storage circuit the encryption key for performing authentication between itself and a mobile communication network at the time of communication connection.

In an encryption key writing apparatus, an inputted encryption key is used to perform a primary operation and while using a code that is the result of the operation, as a secondary encryption key, writing to a mobile wireless telephone device is performed. When performing an encryption operation, the mobile wireless telephone device performs the secondary encryption operation from the secondary encryption key and a random number held in the storage circuit and outputs the result of the operation.