PURPOSE: To recognize the position of voice information that signaling information is multiplexed when conversion is made from asynchronous time division multiplication to synchronous time division multiplication to protect the degradation of the voice quality by generating an index code and adding it to a header area of a cell when converting the synchronous time division multiplication to the asynchronous time division multiplication.

CONSTITUTION: When converting the synchronous time division multiplication to the asynchronous time division multiplication, an index code added to a header area of a cell of the asynchronous time division multiplication indicates the leading position of voice information that signaling information in the voice data area is converted, and generates a signaling multi-position signal representing both the voice information indicated with an index code when converting the asynchronous time division multiplication to the synchronous time division multiplication and the voice information that follows the former voice information at a certain cycle. Accordingly, voice information that signaling information is multiplexed can be recognized, and it can be avoided to threaten to multiplex signaling information independently transmitted in the asynchronous time division multiplication mode to erroneous voice information. With this, the degradation of voice quality can be prevented.