An asynchronous serial communication device in which data transmitted at different speeds coexist and are exchanged. Time losses when errors are generated are effectively eliminated when asynchronous serial data communications for diagnostic failures are performed between a diagnosing tool and an electronic controller via a communications line. In performing the data communications, first data are exchanged between these devices at a low transmission speed and then by switching the speed to high speed various data are exchanged. In a case where communication errors are generated during exchanging, the operation is started again from the data exchanging at the low speed. Here, the edges of the data sampled at the low speed are detected by an edge detecting terminal provided in a communication circuit. A communication error is identified when a difference between a first edge detecting time and a next edge detecting time is shorter than 1 bit time of the low transmission speed data. Based on a further next edge detecting timing, the data receiving by the low speed sampling is started over again.