PURPOSE: To obtain the ineffectualness of signal copy by a method in which an outgoing device is made up of a signal control circuit, a converter to modulation signals, and a level control circuit, and a level judging circuit and a circuit for locking and unlocking by sending out locking control signals only when inputting both deciphering and judging signals are also provided.

CONSTITUTION: Control signals 2 from a signal control circuit 1 are put in a vibrator 3 to send out modulation signals 4, and the signals 4 are transmitted from an outgoing device 17 in the form of transmitting signals 6 whose electric field intensity is kept constant in the level control circuit 5. The signals 6 are received by a receiver 7 to send out a receiving signal 8 and a level signal 9. These are put in a signal deciphering circuit 10. When data are right, a deciphering signal 12 is sent out and put in a level judging circuit 11 where a judging signal 13 is sent out when the electric field intensity is within a fixed range. In the locking control circuit 14, only when both the signals 12 and 13 are input, unlocking is made by locking control signals 15. The locking and unlocking by copied signals can thus be made impossible.