A misfire-detecting system for an internal combustion engine has an ECU which detects a value of a parameter representative of a misfiring state of the engine, and also detects the deterioration degree of a catalytic converter arranged in the engine exhaust system. The ECU sets a misfire-determining reference value, based on the detected deterioration degree of the catalytic converter, compares between the detected value of the above parameter and the misfire-determining reference value, and determines whether the engine is in a misfiring state, based on the result of the comparison.