A receiver sends a first random number to a transmitter. The transmitter generates a sync signal in response to the first random number sent from the receiver. The transmitter embeds the sync signal and key information in a second random number to generate a composite signal. In the composite signal, the sync signal is a position indicator for the key information. The transmitter sends the composite signal to the receiver. The transmitter generates an encryption key from the key information. The receiver detects the sync signal in the composite signal sent from the transmitter. The receiver extracts the key information from the composite signal in response to the detected sync signal. The receiver generates an encryption key from the extracted key information. The encryption key generated by the receiver is equal to that generated by the transmitter. Thus, the transmitter and the receiver hold the same encryption key in common.