PURPOSE: To obtain a cordless telephone set in which a mobile station is simply extended even when a synchronization pattern between a base station and the mobile station differs.

CONSTITUTION: When a new mobile station 3 is extended to a base station 2, radio data including an identification code specifically read from a storage means 5 and a prescribed synchronization pattern set in advance are generated by radio data generating means 6 in the base station 2 and a transmission means 10 sends the radio data to a new mobile station 3. On the other hand, the new mobile station 3 uses a synchronization pattern changeover means 23 to switch the synchronization pattern to be detected into a synchronization pattern of the base station 2 and it is received by a reception means 16. Then a synchronization pattern detection means 17 detects the synchronization pattern included in the received radio data and an identification code detection means 18 detects an identification code included in the received radio data and the detected identification code is written in a storage means 19 to terminate the extension processing.