PROBLEM TO BE SOLVED: To detect variation of air-fuel ratio among cylinders even when one-side delay abnormality occurs in an air-fuel ratio sensor.SOLUTION: An air-fuel ratio imbalance diagnostic apparatus for diagnosing variation of air-fuel ratio among cylinders of an internal combustion engine, includes a target air-fuel ratio setting portion for gradually changing a target air-fuel ratio from a rich side to a lean side, or from the lean side to the rich side, an air-fuel ratio fluctuation amount calculating portion for calculating a fluctuation amount of a sensor value of the air-fuel ratio sensor after the start of the change of the sensor value of the air-fuel ratio sensor in accompany with the change of the target air-fuel ratio, an imbalance determining portion for determining the variation of the air-fuel ratio on the basis of the fluctuation amount.
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