PURPOSE: To connect between networks different in transmission system without defining special connecting system to each other by connecting a busy tone signal converting circuit between a telephone exchange and a digital data network device.

CONSTITUTION: A busy tone signal converting circuit 100 is provided between an analog telephone exchange 200 and a digital data network device 201 without altering connecting interface of the two to change an RG signal 105 and operates ring-off detecting functions possessed by the digital data network device 201. Thus, busy state is transmitted from the telephone exchange 200 to the digital data network device 201. That is, in the case where impossibility of service is reported from the incoming telephone exchange 200, the busy tone is sent through a call line (transmitting line) 101 without changing DL signal 106, and the signal is sent from a busy tone pulse generating circuit 104 to the RG line 105 making the signal on/off in pulselike form.