PURPOSE: To prevent data error, by giving a function to correct the deviation between the bit speed of data received from a terminal station and that by a synchronizing signal in an MODEM, to a data relay device.

CONSTITUTION: Under the condition that the frequency of an MODEM synchronizing signal is set slightly lower than the transmission speed of reception data from a terminal controller 1, the reception data is fetched to a bit buffer 48 serially in synchronizing with the synchronizing signal extracted up from the reception data from a terminal device 3, and the data in the bit buffer 48 is outputted to a line 404 in synchronizing with the MODEM synchronizing signal. The number of bits of the reception data synchronizing signal produced in one- bit interval of an MODEM synchronizing signal 403 is monitored at a phase comparison section 45, and if the reception data synchronizing signal is detected ≥2 bits, the data input to the bit buffer 48 is temporarily stopped immediately after the reception of aport sequence from the terminal device 3 and the speed variation is corrected.