Disclosed is an excessive-signal prevention apparatus which includes a connection state judgment section and a zero-level signal insertion section. The connection state judgment section detects whether connection between a first device and a second device is normal or abnormal on the basis of a pattern of signals that are other than a transmission signal transmitted from the first device to the second device at normal time and are required for transmission of the transmission signal. The zero-level signal insertion section outputs the transmission signal output from the first device to a destination during a period of time when the connection state judgment section judges that the connection between the first and the second device is normal, and which outputs a zero-level signal to the destination during a period of time when the connection state judgment section judges that the connection between the first and the second device is abnormal.