PURPOSE: To eliminate the possibility of malfunction by setting a dangerous period in synchronism recovery operation at the time of the transmission and reception of serial data in consideration of decision deviation of the start position of a stop time or stop period.

CONSTITUTION: A maximum time setting means 4 such as a timer sets the dangerous period corresponding to the maximum time, which can be estimated as the decision deviation of the start position of the stop time or stop period, previously in consideration of the maximum time and a bus controller 1 performs control so that the transmission is started after the dangerous period. Consequently, even a terminal which starts operating at an earlier position than usual does not start transmission in said dangerous period, so the system never malfunctions.