PURPOSE: To execute easily a ciphered data communication to plural other parties by ciphering a data from a data source and transmitting it to other device in accordance with a ciphering key from an information storage medium, and not ciphering a data from a data source when the information storage medium is not inserted.

CONSTITUTION: When a person in charge inserts an IC card 22 into a card inserting port of a data protecting device 21, the card is connected to an IC chip write part 40. Subsequently, when the person in charge inputs a code number by a ten key 27, a control part 26 writes it in an IC chip, compares it with a code number stored in advance, and only when there is a prescribed relation, a signal is sent to a data converting part 31. In this state, the person in charge connects a circuit to a correspondent bank by a telephone, and sends a transaction data to the converting part 31 through a communication control device 13 and an interface part 32 in the device 21 by using a keyboard 3 of a personal computer 1. As a result, the converting part 31 ciphers the data in accordance with a ciphering key from the control part 26, and sends it to the center of the bank through a communication circuit 24.