PURPOSE: To eliminate a need of an asynchronous-synchronous conversion circuit/synchronous-asynchronous conversion circuit in the MODEM device for asynchronous line using a synchronous modulating and demodulating system.

CONSTITUTION: A stop bit detecting means is provided which consists of a flip-flop circuit 25, which delays a data write signal W by one bit and supplies a stop bit discrimination signal BR, and a counter 26 which counts the stop bit in response to the signal BR up to a set number. The stop bit continuity state that the set number of or more stop bits are continuous is detected. A reset means is provided which is provided with a switch circuit 27 which selects the signal BR as a reset signal R in the case of the counted value of the counter 26 smaller than the set number but selects the data write signal W as the reset signal R in the case of the counted value equal to or larger than the set number.