PURPOSE: To make it possible to omit labor for distributing floppy disks in conventional technique and manually converting a cipher code at previously determined time and to simultaneously rewrite data by plural terminals.

CONSTITUTION: An ISDN interface circuit 8 receives a cipher key code sent by a D-channel user area on an ISDN line and sends the received code to a control circuit 5 through a D-channel control circuit 7. After checking the normality of the cipher code, the control circuit 5 rewrites a cipher code in a storage area of a built-in memory. When a cipher communication execution request is inputted from an I/O circuit 6 prior to communication, the control circuit 5 commands the control circuit 7 to adjust the cipher communication execution request to the D-channel and send the request to an opposite terminal through an ISDN interface circuit 8. At the time of receiving a cipher communication accepting signal from the opposite terminal through the D-channel, a conversion signal based upon the stored cipher key code is converted into a cipher signal by a ciphering circuit 3.