A catalyst deterioration determination device capable of determining whether or not fuel is high-sulfur fuel containing much sulfur content, and properly determining whether a catalyst is deteriorated while suppressing the frequency of execution of control for recovering the catalyst from the poisoned state to the minimum. The device determines whether or not a catalyst which purifies exhaust gases exhausted from an internal combustion engine is deteriorated based on the capacity of the catalyst for purifying exhaust gases. If it is determined that the catalyst is deteriorated, first sulfur elimination control is executed for eliminating sulfur content accumulated in the catalyst. Further, when the first sulfur elimination control is terminated, the deterioration determination of the catalyst is executed. Then, when it is determined by the deterioration determination that the catalyst is not deteriorated, it is determined that the fuel is high-sulfur fuel containing lots of sulfur content.