To provide a technique by which the occurrence of a malfunction, such as the radio interference, etc., can be prevented in a hand-free device for portable telephone.

In the hand-free device 11, a sequence number is given to a packet transmitted between a host machine 15 and a slave machine 21. Since the sequence number has an initial value taken at the time when power is supplied, increases at prescribed time intervals until an incoming call arrives, and, after arriving, increases whenever a real packet is transmitted or received, the sequence number of the packet hardly becomes equal to that of another packet transmitted from or received by another speaking system and, accordingly, the malfunction such as the radio interference, etc., hardly occurs even when a plurality of packets is transmitted. Consequently, the occurrence of the radio interference can be prevented at a low cost as compared with the case where packets are identified by giving an individual identification number to each speaking system.