DEVICE FOR JUDGING IMBALANCE OF AIR/FUEL RATIO AMONG CYLINDERS OF MULTICYLINDER INTERNAL COMBUSTION ENGINE

A determining apparatus comprises a catalytic converter 43, an upstream air-fuel ratio sensor 55 including an air-fuel ratio detecting element covered by a diffusion resistance layer, and a downstream air-fuel ratio sensor 56. The determining apparatus performs a main feedback control to cause an air-fuel ratio represented by the output value of the upstream air-fuel ratio sensor to coincide with a target upstream-side air-fuel ratio. The determining apparatus performs a sub feedback control to cause the output value of the downstream air-fuel ratio sensor to coincide with a target downstream-side value. The determining apparatus obtains "an imbalance determining parameter" which becomes larger, as "a difference becomes larger between "an amount of hydrogen included in the exhaust gas before passing through the catalytic converter" and "an amount of hydrogen included in the exhaust gas after passing through the catalytic converter", and determines that an air-fuel ratio imbalance among cylinders is occurring, when the imbalance determining parameter becomes equal to or larger than an abnormality determining threshold. The determining apparatus does not perform the determination of the air-fuel ratio imbalance among cylinders, when a prohibiting condition for the determination is satisfied, such as when an exhaust gas rate is larger than a predetermined value.