PURPOSE: To change a key automatically, by sending out a synchronous pattern including the change command information of a key data file consisting of random numbers, using a new key data file after sending out the said pattern, and also using a new key data file in the receiving side after receiving the said synchronous pattern.

CONSTITUTION: In case a key is changed, a key change time signal is sent out to the timing circuit 2 and the key No. store circuit 3 from the time designating circuit 1. An output from the key change command pattern generating circuit 10 is provided to the output circuit 8 by an output of the circuit 2, also an output of the key data circuit 5 is changed to a random number 6 by an output from the circuit 3, is provided to the circuit 8, and these outputs are sent out to the receiving side by the circuit 8. On the other hand, in the receiving side, a key change time signal is provided to the timing circuit 2' and the circuit 3 from the circuit 1, the circuit 2' provided a receiving key load trigger to the circuit 5, and the circuit 3 provides a key No. corresponding to a time to the circuit 5. The circuit 6 supplies the key data signal to the random number generating circuit 6.