To use a telephone set in connection with a public line through a cable through the user of the transmitter-receiver of a mobile telephone terminal by providing a line connecting means for connecting the mobile telephone terminal connected through an electrically connecting means to the public line.

A public telephone system 50 is provided with a display 51, ten keys 52, a charging device 53, a power source device 54, a public network connector 55, a signal processor 56, a private transmitter/receiver 57, a signal input/output device 58, a charge designating button 59 and connectors 60 to 62. A portable telephone set 10 can be connected to each of the three connectors 60 to 62 provided at this system 50 though a connecting cable 14. Instead of the set 10, a PHS terminal can be connected. Then, in this connector 55, this system 50 and the set 10 connected to the connectors 60 to 62 are connected with an exchange 20 through a public network such as an ISDN line respectively.