An electronic control unit has (i) a readiness storage area storing readiness indicating that an abnormality diagnosis for diagnosing an abnormality diagnosis item is complete, and (ii) a microcomputer clearing all readiness upon receiving an all clear request based on a condition that is different from a reception of the all clear request. In such configuration, the all clear request for clearing all readiness in the readiness storage area is appropriately handled by the electronic control unit.