PURPOSE: To avoid errorneous recognition at an incoming terminal by permitting sound and FAX data to constitute individual frames and preventing switching noise even at the time of transmission when they are individually synchronized and even when the same frame is used.

CONSTITUTION: A FAX repeat transmission part 202 recognizes an input signal 220 to be FAX data and transmits it to a sound encoding part 201 through a transmission switching part 203. The encoding part 201 transmits a switching notice in a means which does not affect previously decided sound to a second device 5 as a signal 240 by a private line 9 through a transmission switching part 204. The switching notice is detected in a sound decoding part 209 and sound is made silent and is noticed to a transmission switching control part 213. The control part 213 transmits a response to a sound encoding part 215 and transmits a notice signal 230 from a transmission switching part 214 to the reception-side of a first device 4 through the transmission line 9. A sound decoding part 207 detects the reception of the response and sound is made silent. Thus, occurrence of noise can be prevented at the time of switching sound to FAX.