Copyright of contents to be transmitted may be protected for an analog signal produced by converting a digital signal representative of the contents. A signal transmission apparatus determines whether the contents are protected by copyright by detecting copyright protection information of a digital image signal of the contents at a protection information detecting unit, and scrambles an analog image signal converted from the digital image signal to output when the copyright is protected, or prohibits outputting of the analog image signal. When a receiving apparatus receives a transmitted signal, the apparatus generates a digital image signal and the analog image signal based on the transmitted signal. When the authentication processing unit of the receiving apparatus has determined that the copyright has been protected through communication between an authentication processing unit of the transmission apparatus and that of the receiving apparatus, the unit scrambles the analog image signal to output, or prohibits outputting thereof, or superimposes a-copy-guard signal on the image signal to output.