PURPOSE: To improve the convenience of use by continuing speaking by switching a mobile (portable) telephone set to a portable telephone set through a radio line while that mobile telephone set is used as a cable telephone set.

CONSTITUTION: A line is connected to an exchange station 13 by operating a cable telephone processing circuit, and speaking with a cable telephone set 14 at the call originating destination is performed. During this speaking, a line selection switch is depressed, and a specified signal (ID) showing the use of a mobile station 15 as a radio telephone set and a telephone number registered on this mobile telephone system are transmitted through a base station 12 to a mobile telephone control station 11. The mobile telephone control station 11 connects the radio line between the mobile station 15 and the base station 12 and next controls the connection/disconnection of a cable telephone line between the relevant mobile station 15 and the exchange station 13. Afterwards, the mobile station 15 is operated as the radio telephone set, its speaking is continued, and a mobile station side connecting connector 17 is detached from a connector 16 for line connection so that the mobile station can move outdoors or the like.