PURPOSE: To easily detect the error of a timing clock used at a data terminal device side and a modulator/demodulator side, by extracting a relative timing between a transmission timing signal and transmission data, and sending a synchronization error signal when large deviation exists between them.

CONSTITUTION: The titled device is provided with an oscillator 1 which oscillates the transmission timing signal, a clock selection circuit 2 which switches a clock, a clock extraction circuit 3 which detects and outputs the change point of the clock, a data extraction circuit 4 which detects and outputs the change point of the transmission data, an error detection circuit 5 which detects a synchronization error, and a display circuit 6 which displays the synchronization error. Then, the change point of the transmission data and that of a transmission data timing signal of the transmission data from a data terminal device are extracted, and the relative timing between the change points of extracted transmission data and transmission data timing signal is monitored, and the synchronization error is set when the above timing exceeds a prescribed value. In such a way, it is possible to easily detect the error of the timing clock used at the data terminal device side and the modulator/demodulator side.