PURPOSE: To omit the pull in period at reception by repeating the transmission and reception to synchronize the transmission timing and a timing generator as if signal were apparently continuously received.

CONSTITUTION: While a carrier is being received, a switch 22 is intermitted to detect the error of the transmission timing of a reception signal and the output timing if a timing generator 19 is synchronized always with the data transmission timing of the reception signal while the transmission/reception is changed over by controlling timing of a timing generator 20. Thus, it is not required to provide a time for the pull in of the transmission timing, and the two-way transmission is attained with considerable good efficiency.