An on-vehicle electronic control device includes an auxiliary microprocessor and subjects a microprocessor allocated to a main part of control to an external diagnosis, thereby improving reliability of performance. A microprocessor including a nonvolatile program memory into which a control program is written is serially connected to an auxiliary microprocessor including an auxiliary nonvolatile program memory. The microprocessor and the auxiliary microprocessor function in cooperation to control on-vehicle electric load groups in response to input signals from on-vehicle sensor groups and on-vehicle analog sensor group. The nonvolatile program memory and the microprocessor are subjected to runaway monitoring performed by a watchdog timer and to an external checksum diagnosis performed periodically by the auxiliary microprocessor. If an anomaly occurs in the runaway monitoring, the external checksum diagnosis, and a checksum interval, parts of electric loads are cut off of power supply by load power relay.