A judging device comprises a catalyst, an upstream air/fuel ratio sensor having an air/fuel ratio sensing element covered with a diffusion resistance layer, and a downstream air/fuel ratio sensor. The judging device performs main feedback control to equalize the air/fuel ratio indicated by the output value of the upstream air/fuel ratio sensor to an upstream target air/fuel ratio and sub-feedback control to equalize the output value of the downstream air/fuel ratio sensor to a downstream target value. The judging device acquires “an imbalance judging parameter” which increases with “the increase of the difference between the amount of hydrogen contained in the exhaust gas before the exhaust gas passes through the catalyst and that after the exhaust gas passes through the catalyst” according to the sub-feedback amount. When the imbalance judging parameter is larger than an abnormality judgment threshold, the judging device judges that an air/fuel ratio imbalance among the cylinders has occurred. The judging device does not make judgment on air/fuel ratio imbalance among the cylinders if a predetermined judgment prohibition condition is satisfied, for example, if the flow of the exhaust gas is a predetermined value or more.