A catalyst deterioration-detecting device for an internal combustion engine has at least one catalyst arranged in an exhaust passage of the engine,for purifying exhaust gases emitted from the engine, first and second exhaust component concentration sensors arranged in the exhaust passage upstream and downstream of the catalyst, respectively. The catalyst deterioration-detecting device calculates an average value of an output value from the second exhaust component concentration sensor, calculates an output fluctuation width with respect to the average value, based on the output value from the second exhaust component concentration sensor, and determines whether the catalyst is deteriorated, based on the calculated average value and the calculated output fluctuation width. The detection of the catalyst deterioration is carried out when the engine is in a predetermined operating condition defined by engine temperature, engine speed and engine load.