Abnormality diagnostic method for internal combustion engine and abnormality diagnostic device for internal combustion engine

A device for diagnosing an abnormality of an internal combustion engine, comprising:

• a first intake throttle valve for controlling an intake air amount of the internal combustion engine;
• a second intake throttle valve positioned on an upstream side of the first intake throttle valve;
• an intake air amount detecting sensor positioned on an upstream side of the second intake throttle valve and configured to detect the intake air amount;
• a blow-by gas recirculation system for blow-by gas treatment connected to a position more on a downstream side than the intake air amount detecting sensor in an intake passage;
• and
• a diagnosis section configured to diagnose an abnormality in the blow-by gas recirculation system, on a basis of a first intake air amount detected by the intake air amount detecting sensor when an opening degree of the second intake throttle valve is set to a predetermined first valve opening degree and a second intake air amount detected by the intake air amount detecting sensor when the opening degree of the second intake throttle valve is set to a predetermined second valve opening degree smaller than the first valve opening degree, during a predetermined operation state in which an opening degree of the first intake throttle valve is fixed,
• wherein, during detecting the first intake air amount and the second intake air amount, when the opening degree of the first intake throttle valve is varied, the diagnosis for the blow-by gas recirculation system is stopped.