PURPOSE: To easily conduct ciphered communication.

CONSTITUTION: A caller side generates a secret key based on its own telephone number and a telephone number of a called party, then ciphers image data based on the secret key and sends the ciphered data. The called party receives data as a modem signal including a telephone number of the caller sent for a call signal bell stop period. Then the caller telephone number is detected from the received data and generates a secret key based on the telephone number and its own telephone number. Upon the receipt of the ciphered data, the called party decodes the ciphered data based on the secret key and prints out the data.