An electronic control apparatus is provided in which a supervisory control circuit part capable of mutual communication with a main control circuit part, which is operated by a high speed clock signal of high precision, is operated by a medium speed clock signal, which is low in cost and precision, such that they can operate mutually independently of each other. The main control circuit part supplies a medium speed communication synchronization signal frequency divided to the supervisory control circuit part. The supervisory control circuit part is operated by the medium speed clock signal to supervise the control operation of the main control circuit part. An error measuring circuit calculates an error correction coefficient which is the ratio of the signal cycle of the communication synchronization signal to that of the medium speed clock signal. The value of the error correction coefficient is reversely supervised by the main control circuit part.