KEY TELEPHONE SYSTEM

PURPOSE:To promptly and surely grasp an abnormal state without going to a place where a sensor is provided by making the state between the hands free telephone set of a child telephone set provided relationally to the sensor and the child telephone set into an extension connected state when the child machine in which an alarm tone is ringing is hooked off. CONSTITUTION:Alarm tones are generated from respective child telephone sets 41-4n of a key telephone set through a main device 1 according to the detection of abnormality by a sensor 5. When the child telephone set 41, etc., are hooked off under this state, while the alarm tones are stopped, the child telephone set 41 and the hands free telephone set provided for the child telephone set 41 relationally to the sensor 5 are extension-connected, and a tone from the sensor 5 can be heard at the child telephone set 41. Consequently, the abnormal state can be grasped without going to the place where the sensor is provided, and processing for the abnormality can be promptly and surely executed.