PURPOSE: To connect a single telephone set to a key telephone master set and to use a function of the key telephone master set from the single telephone set.

CONSTITUTION: When an incoming call signal of a doorphone is detected by a doorphone call detection circuit 2, an incoming call signal is sent to a key telephone set through a door incoming call information data communication circuit 4 from a CPU 5, a pseudo incoming call transmission circuit 9 informs it to the single telephone set and a doorphone incoming call detection circuit 2 controls a doorphone relay to connect the doorphone to the single telephone set and the key telephone set and the reply of the pushbutton telephone set to the doorphone is informed through the data communication circuit 4 and the reply of the single telephone set is informed through a DT, PB signal identification circuit 8 to a CPU respectively.