The invention provides an inter-cylinder air/fuel ratio imbalance determination apparatus and method. The determination apparatus includes a “limiting current type air/fuel ratio sensor”, and acquires a pre-correction index quantity that is greater the greater the degree of non-uniformity of the cylinder-by-cylinder air/fuel ratios, on the basis of a time differential value of the output value of the air/fuel ratio sensor. The determination apparatus obtains as the correction-purpose output value an average value of the output value obtained during a fuel-cut operation. The correction-purpose output value is greater the higher the responsiveness of the air/fuel ratio sensor. The determination apparatus acquires an air/fuel ratio imbalance index value by correcting the pre-correction index quantity so that the pre-correction index quantity is smaller the greater the correction-purpose output value. It is determined that an inter-cylinder air/fuel ratio imbalance state has occurred, when the air/fuel ratio imbalance index value is greater than or equal to an imbalance determination threshold value.