In an electronic control unit, a control constant stored in a program memory is transmitted to a data memory via a serial communication circuit. First control circuit section (master station) and second control circuit section (substation) mutually communicate through series-parallel converters. The master station includes regular transmission device and storage constant confirmation processing device. The substation includes regular report device and confirmation reply device to a data transmitted from the master station. A constant set data transmitted from the master station by regular transmission device is stored in the data memory at the time of normal receiving. Confirmation information such as sum value of the entire data in the data memory is reported by regular report device. At the master station, if there is any error in the confirmation information, a constant set data is retransmitted with regular transmission device. Reliability of data in the data memory is improved.