PURPOSE: To attain the start-stop type sinchronism with high reliability, by determining the time width of a start polarity period from a synchronizing pattern code where the start polarity period transferred from the transmission side and a stop polairty period appear repetitively, for establishing the synchronism.

CONSTITUTION: A start polarity detecting section 1 outputs a counter count indicating signal and a start detecting end signal, and the counter count indicating signal is applied to a counter 2 during the start polarity period and the counter 2 decides the pulse width of the start polarity period based on this signal. Further, the pulse width information of this period is given to the counter 2 and set to a register 3. Further, the content of the register 3 is applied to a transmission speed detecting section 4. The detecting section 4 comprises an ROM and the like and generates a transmission speed indicating signal (j) most suitable to the transmission speed at the transmission side after establishing the synchronism.