Abnormality diagnostic device and abnormality diagnostic method for air-fuel ratio sensor

An abnormality diagnostic device for an air-fuel ratio sensor that detects an air-fuel ratio of an exhaust gas of an internal combustion engine, comprising:

• an identification unit that models a system extending from a fuel injection valve to the air-fuel ratio sensor by using a first order response delay element, and that identifies a parameter of the first order response delay element based on an input air-fuel ratio given to the air-fuel ratio sensor which occurs when the input air-fuel ratio is relatively sharply changed in accordance with an engine operation requirement, and an output of the air-fuel ratio sensor that changes in response to a change in the input air-fuel ratio;
• and
• an abnormal determination unit that determines an abnormality of a predetermined characteristic of the air-fuel ratio sensor based on the parameter identified by the identification unit.