PURPOSE: To provide an error processing method in serial communication capable of executing a fail operation at an appropriate timing.

CONSTITUTION: A target opening arithmetic CPU 5 and a sub throttle driving CPU 6 are connected mutually with an eight-bit communication line 7. Communication errors such as a placing error, an over-run error, a noise flag, and a parity error are considered in the serial communication, and the error except the noise flag are significant. Weighing is applied in advance corresponding to the significance of the error in the serial communication. When the error is detected in the serial communication, a count-up operation is performed as counting up the count value of a fail operation execution counter by the share of weighing, and also, when no error is detected for a prescribed period, the count value is counted down, and when the count value arrives at a prescribed value (=(13)), the fail operation is executed.