When transmitting serial data from a master device to a slave device, it is possible to promptly detect a communication error if any occurs. Serial data transmitted from the master device to the slave device has two or more continuous bytes of dummy data having an identical structure. When the slave device recognizes the dummy data, communication error processing is executed. Assume that the serial data is shifted by an affect of a noise. In this case, “a text end control code (ETX)” is also shifted and the serial data cannot be recognized and no data reception end process is executed. However, during a period after this, a part of the first dummy data and a part of the second dummy data are received and one dummy data is recognized. Thus, the slave device can promptly execute the communication error processing.