An abnormality determination apparatus and method for a blow-by gas feedback device, and an engine control unit are provided for improving the accuracy of abnormality determination even when an intake air amount controller is provided in an intake system at a location downstream of a joint of a blow-by gas passage. The blow-by gas feedback device feeds a blow-by gas back to an intake system of an internal combustion engine through a blow-by gas passage connected to the intake system through a joint. The abnormality determination apparatus comprises an intake air amount sensor provided in the intake system at a location upstream of the joint for detecting an intake air amount, an intake air amount controller provided in the intake system at a location downstream of the joint for controlling the intake air amount, and an ECU for determining an abnormality of the blow-by gas feedback device based on the intake air amount detected by the intake air amount sensor after the intake air amount has changed due to the operation of the intake air amount controller.