To provide a catalyst deterioration state evaluation system for an internal combustion engine, capable of improving accuracy of evaluation of a catalyst deterioration state and suppressing deterioration of emission.

Based on a detection value of a sub-O2 sensor in a catalyst downstream side provided in an exhaust system of the internal combustion engine, an air fuel ratio in a catalyst upstream side of the internal combustion engine is forcibly set to be rich or lean, to evaluate the deterioration state of the catalyst. After the sub-O2 sensor outputs a rich or lean detection value, when a suction air integrated value (integration counter value of the amount of exhaust gas passing through the catalyst) egasum until the output is inverted reaches a predetermined value Ga, the air fuel ratio control is inverted such that the air fuel ratio in the catalyst upstream side becomes lean or rich.