A monitoring apparatus for an internal combustion engine, applied to a multi-cylinder internal combustion engine having a plurality of cylinders comprising: a fuel injector for injecting fuel; a catalytic converter disposed in an exhaust passage of said engine and at a position downstream of an exhaust gas aggregated portion into which exhaust gases discharged from combustion chambers of a plurality of said cylinders of said engine merge; an upstream air-fuel ratio sensor, disposed in said exhaust passage and at said exhaust gas aggregated portion or between said exhaust gas aggregated portion and said catalytic converter, and outputting an output value (Vabyfs) corresponding to an air-fuel ratio of a gas flowing at a position at which said upstream air-fuel sensor is disposed; a downstream air-fuel ratio sensor, disposed in said exhaust passage at a position downstream of said catalytic converter, and outputting an output value (Voxs) corresponding to an air-fuel ratio of a gas flowing at said position at which said downstream air-fuel sensor is disposed; sub feedback amount calculation means for calculating a sub feedback amount to make an air-fuel ratio represented by said output value of said downstream air-fuel ratio sensor coincide with a stoichiometric air-fuel ratio, every time a predetermined first update timing arrives; fuel injection control means for controlling an injection amount of fuel injected from said fuel injector every time a predetermined second update timing arrives, using at least said output value (Vabyfs) of said upstream air-fuel ratio sensor and said sub feedback amount in such a manner that an air-fuel ratio of an air-fuel mixture supplied to said engine coincides with the stoichiometric air-fuel ratio; learning means for updating a learning value of said sub feedback amount every time a predetermined third timing arrives by making said learning value of said sub feedback amount approach a convergent value of said sub feedback amount; monitoring means for performing an air-fuel ratio imbalance determination as to whether or not an air-fuel ratio imbalance among cylinders is occurring, by comparing a threshold (Ath) and a first parameter (Avefsfbg) for an air-fuel ratio imbalance determination, said first parameter (Avefsfbg) being an average of said learning value; learning value updating rate setting means for setting an updating rate of said learning value at any one of a firstupdating rate, a second updating rate smaller than said first updating rate, and a third updating rate smaller than said second updating rate; and monitoring control means for allowing or prohibiting said monitoring means to perform said air-fuel ratio imbalance determination depending on said set updating rate of said learning value. a fuel injector for injecting fuel; a catalytic converter disposed in an exhaust passage of said engine and at a position downstream of an exhaust gas aggregated portion into which exhaust gases discharged from combustion chambers of a plurality of said cylinders of said engine merge; an upstream air-fuel ratio sensor, disposed in said exhaust passage and at said exhaust gas aggregated portion or between said exhaust gas aggregated portion and said catalytic converter, and outputting an output value (Vabyfs) corresponding to an air-fuel ratio of a gas flowing at a position at which said upstream air-fuel sensor is disposed; a downstream air-fuel ratio sensor, disposed in said exhaust passage at a position downstream of said catalytic converter, and outputting an output value (Voxs) corresponding to an air-fuel ratio of a gas flowing at said position at which said downstream air-fuel sensor is disposed; sub feedback amount calculation means for calculating a sub feedback amount to make an air-fuel ratio represented by said output value of said downstream air-fuel ratio sensor coincide with a stoichiometric air-fuel ratio, every time a predetermined first update timing arrives; fuel injection control means for controlling an injection amount of fuel injected from said fuel injector every time a predetermined second update timing arrives, using at least said output value (Vabyfs) of said upstream air-fuel ratio sensor and said sub feedback amount in such a manner that an air-fuel ratio of an air-fuel mixture supplied to said engine coincides with the stoichiometric air-fuel ratio; learning means for updating a learning value of said sub feedback amount every time a predetermined third timing arrives by making said learning value of said sub feedback amount approach a convergent value of said sub feedback amount; monitoring means for performing an air-fuel ratio imbalance determination as to whether or not an air-fuel ratio imbalance among cylinders is occurring, by comparing a threshold (Ath) and a first parameter (Avefsfbg) for an air-fuel ratio imbalance determination, said first parameter (Avefsfbg) being an average of said learning value; learning value updating rate setting means for setting an updating rate of said learning value at any one of a firstupdating rate, a second updating rate smaller than said first updating rate, and a third updating rate smaller than said second updating rate; and monitoring control means for allowing or prohibiting said monitoring means to perform said air-fuel ratio imbalance determination depending on said set updating rate of said learning value.