An electronic control unit includes a first microprocessor monitoring the operation of a second microprocessor and has a monitoring operation blocking unit for preventing the second microprocessor from being reset by the first microprocessor while the second microprocessor is loading from a memory thereof so that even when a control program is loaded into the monitoring microprocessor before one is loaded into the monitored microprocessor, the control program can be loaded with certainty. Each microprocessor executes a control program stored in the memory, and when a predetermined reloading condition has been established, executes a loading process for receiving load data transmitted thereto from outside into the memory. The microprocessors use the same single communication line to conduct communication with the outside and the electronic control unit further includes a communication operation controlling unit for, when one of the microprocessors is loading, prohibiting the other microprocessors from transmitting data to the communication line. In this way, it is possible to reload a control program and control data into any of the microprocessors without the data being destroyed by data from the other microprocessors.