PURPOSE: To replace interfaces without the loss of data by transmitting device state such as ready state, transmission requesting state and transmission end state between current and voltage interfaces.

CONSTITUTION: A conversion circuit between a current interface in a TELEX terminal and a CCITT, V24 interface in a TELEX terminal control device is provided with a current value observing circuit 11 for detecting the opening of a transmission side line SL of the TELEX terminal and a current value/voltage value converting circuit 14 connected to a receiving side line RL of the TELEX terminal. A signal control circuit 12 is connected between the control device and the circuit 11 to recognize the non-ready state or transmission request of the TELEX terminal of the basis of the detection of the state of the line SL by the circuit 11. A specific code sending circuit 13 is also connected between the node of the circuits 11, 12 and the circuit 14. Thus, the device state such as the ready state, the transmission request state and the transmission end state is transmitted between the TELEX terminal and the control device and the interfaces are replaced without the loss of data.