When an internal combustion engine including a plurality of cylinders is operating steadily, an index value relating to an actual hydrogen content in exhaust gas downstream of a portion where exhaust passages from the cylinders merge, and upstream of a catalyst is detected. When an index value relating to the actual hydrogen content in the exhaust gas is larger than a determination index value relating to a hydrogen content corresponding to a permissible limit of air-fuel ratio variation, it is determined that there is abnormal air-fuel ratio variation between the cylinders.