PURPOSE: To transmit a main signal and a sub signal by using the transmission line of single channel and to improve transmission efficiency by providing a signal forming means for main transmission, a signal forming means for sub transmission, and a signal switching control means.

CONSTITUTION: Error detection code attaching parts 12a, 12b which perform the generation and addition of error codes by different generating functions on each output of an audio encoder 11 and a signaling encoder 15a, and a switching circuit SW which selects the output of the attaching parts 12a, 12b according to a signaling flag and supplies it to a scrambling part 13 are provided. The main signal, after an arithmetic operation by a prescribed generating polynominal for error detection being performed when the sub signal remains unchanged, is outputted to the transmission line as a frame. When the sub signal is changed, the arithmetic operation is performed on the sub signal by the generating polynominal different from the above-mentioned generating polynominal, and is generated as the frame, and the frame is outputted to the transmission line instead of the main signal at that time. Thereby, it is possible to transmit the main signal and the sub signal via a single transmission line, and to improve the transmission efficiency.