PURPOSE: To prevent interruption of a telephone line by an erroneous operation at the time of transfer.

CONSTITUTION: This telephone set is composed of a master unit 2 and a slave unit 3, the master unit 2 is provided with one of plural communication units and a line control means and the slave unit 3 is provided with the other communication units. At the time of performing the transfer from the master unit 2 performing outside line communication to the slave unit 3, in response to slave unit calling response signals by the response operation of the slave unit 3 which responds to slave unit call signals by a call operation in the master unit 2, the telephone line 14 is connected to a transmission circuit 6 and a reception circuit 7 in the master unit 2. Also, a timer 1 starts timing. When on-hook signals by an on-hook operation from the slave unit 3 are received within a predetermined period, the on-hook signals are invalidated and the telephone line 14 is not interrupted.