Provided is an electronic control unit capable of performing a high-accurate failure diagnosis in a short time as an electronic control unit having a failure diagnosis function. A load drive circuit that drives a load, a control circuit that controls the load drive circuit, and a diagnosis circuit that diagnoses an output state of the load drive circuit and outputs a diagnostic result to the control circuit are provided. The diagnosis circuit outputs a diagnosis completion flag indicating whether or not there is a diagnosis opportunity to the control circuit.