The present invention provides a data transmission apparatus which enhances the safety with respect to leakage of encoded transmitted data by raising the safety of transmission of an encoding key or a decoding key. It provides a large capacity transmission line 152 such as a satellite transmission line for transmitting a large amount of data and a small capacity transmission line 156 such as a public telephone line for transmitting an encoding key or decoding key or information for the generation of them. The encoding session key 107 is generated by using a work key and a destination address of a receiving apparatus of the destination. Further, the decoding session key 108 is generated using one's own address and the work key. The work key is preferably encoded and transmitted through the small capacity transmission line 156. In the transmitting apparatus 101, in addition to the encoding of the data 105 which should be transmitted by using the encoding session converted as described above, transmission control information such as destination data is added and is transmitted to the receiving apparatus 102 through the large capacity transmission line 152. In the receiving apparatus 102, the encoded data 110 is decoded by using the decoding session key 108 converted as described above.