according to the present invention , the operation of a diagnostic circuit within a load drive circuit , which has been built into an electronic control device , is checked without modifying the electronic control device or the like . the load drive circuit , which drives a load such as a solenoid using a dc power supply as a power supply , includes a drive circuit and a diagnostic circuit which is independently provided within the drive circuit . upon input of a drive - stop signal from a control circuit that controls the drive circuit , the operation of at least the drive circuit stops , so that whether the diagnostic circuit , which diagnoses the condition of the load , is normally operating or not is checked in a condition in which the load drive circuit has been built into the electronic control device .