To enable the data communication of communication equipment such as a personal computer through a mobile telephone set by enabling automatic dialing to a mobile telephone line by connecting a modem circuit for cable telephone line to the mobile telephone set, detecting a telephone number outputted from the modem, analyzing that signal and transmitting it to the mobile telephone set.

A telephone system detecting part 12 detects a currently connected line and determines a telephone system and when the communication through a mobile telephone set 4 is selected, a controller 13 grasps a dial number from a modem part 7 and sends dial information and a dial command to the mobile telephone set 4. The mobile telephone set 4 transmits a dial to a mobile telephone line 6 and after the opposite side is connected, an automatic data communication line with communication equipment 1 such as a personal computer is constituted. Besides, the controller 13 monitors the report of call incoming from the mobile telephone set 4 so that the modem part 7 can be let perform a call incoming response and after the call incoming response, data communication can be started.