An engine has a PCV passage connected to an intake system for recirculating a blow-by gas to the intake system and an air flow meter provided upstream of a connection between the PCV passage and the intake system in the intake system for sensing an intake air flow rate. An electronic control unit (ECU) performing various types of control of the engine diagnoses a clogging abnormality of the PCV passage based on pulsation width of the intake air flow rate sensed with the air flow meter. A thermal type sensor having a heating section and a temperature sensing section provided on a semiconductor substrate is used as the air flow meter.