PURPOSE: To attain the application as an on-car radio communication device, by attaching a required antenna, an amplifier and a power supply to a container and connecting said container to a portable radio telephone set with a spiral cable.

CONSTITUTION: The portable radio telephone set 1 uses an antenna 2 at an antenna switching section 16, functions as an independent radio telephone set by removing a connector 6 but does not have sufficient performance as an on-car device. In inserting a connector 6 to the telephone set 1, the set 1 is connected to the container 4 via the spiral cable 3, an output of a section 15 using antenna in common is connected to a section 17 using antenna in common of the container 4 at an antenna switching section 16, a required transmission output and receiving sensitivity is provided additionally with a transmission amplifying section 18 and a reception amplifying section 19 and connected to an antenna from a section 20 using antenna in common. Thus, the performance suitable for the on-car set is provided.