A diagnosis system of an internal combustion engine comprising an exhaust purification catalyst arranged in an exhaust passage of the internal combustion engine and being able to store oxygen in inflowing exhaust gas and an air-fuel ratio sensor arranged at a downstream side of the exhaust purification catalyst in a direction of exhaust flow and detecting an air-fuel ratio of exhaust gas flowing out from the exhaust purification catalyst, stopping or decreasing a feed of fuel to a combustion chamber as fuel cut control, and controlling an air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst after the end of the fuel cut control to a rich air-fuel ratio richer than a stoichiometric air-fuel ratio as post reset rich control, wherein the diagnosis system comprises a first change characteristic calculating means for calculating a first characteristic of change of air-fuel ratio at the time the output air-fuel ratio of the air-fuel ratio sensor first passes a first air-fuel ratio region which is a part of an air-fuel ratio region of a stoichiometric air-fuel ratio or more, after an end of the fuel cut control, based on an output air-fuel ratio output from the air-fuel ratio sensor, a second change characteristic calculating means for calculating a second characteristic of change of air-fuel ratio at the time the output air-fuel ratio of the air-fuel ratio sensor first passes a second air-fuel ratio region different from the first air-fuel ratio region, after an end of the fuel cut control, based on an output air-fuel ratio output from the air-fuel ratio sensor, and an abnormality diagnosing means for judging any one of normality, abnormality, and whether a hold should be put on judgment for the state of the air-fuel ratio sensor, based on the first characteristic of change of air-fuel ratio and, if it is judged based on the first characteristic of change of air-fuel ratio that a hold should be put on judgment, for judging if the state of the air-fuel ratio sensor is normal or abnormal based on the second characteristic of change of air-fuel ratio, wherein the first air-fuel ratio region includes an air-fuel ratio region leaner than the second air-fuel ratio region, and wherein the second air-fuel ratio region includes an air-fuel ratio region richer than the first air-fuel ratio region, the diagnosis system being characterized in that: the first characteristic of change of air-fuel ratio is a first speed of change of air-fuel ratio which is a speed of change when an output air-fuel ratio of the air-fuel ratio sensor first passes through the first air-fuel ratio region, and the abnormality diagnosing means judges that the air-fuel ratio sensor is abnormal when the first speed of change of air-fuel ratio is slower than a speed of change used as reference for abnormality, judges that the air-fuel ratio sensor is normal when the first speed of change of air-fuel ratio is faster than a speed of change used as reference for normality, and judges that a hold should be put on judgment when the first speed of change of air-fuel ratio is between the speed of change used as reference for abnormality and the speed of change used as reference for normality. wherein the diagnosis system comprises a first change characteristic calculating means for calculating a first characteristic of change of air-fuel ratio at the time the output air-fuel ratio of the air-fuel ratio sensor first passes a first air-fuel ratio region which is a part of an air-fuel ratio region of a stoichiometric air-fuel ratio or more, after an end of the fuel cut control, based on an output air-fuel ratio output from the air-fuel ratio sensor, a second change characteristic calculating means for calculating a second characteristic of change of air-fuel ratio at the time the output air-fuel ratio of the air-fuel ratio sensor first passes a second air-fuel ratio region different from the first air-fuel ratio region, after an end of the fuel cut control, based on an output air-fuel ratio output from the air-fuel ratio sensor, and an abnormality diagnosing means for judging any one of normality, abnormality, and whether a hold should be put on judgment for the state of the air-fuel ratio sensor, based on the first characteristic of change of air-fuel ratio and, if it is judged based on the first characteristic of change of air-fuel ratio that a hold should be put on judgment, for judging if the state of the air-fuel ratio sensor is normal or abnormal based on the second characteristic of change of air-fuel ratio, the first characteristic of change of air-fuel ratio is a first speed of change of air-fuel ratio which is a speed of change when an output air-fuel ratio of the air-fuel ratio sensor first passes through the first air-fuel ratio region, and the abnormality diagnosing means judges that the air-fuel ratio sensor is abnormal when the first speed of change of air-fuel ratio is slower than a speed of change used as reference for abnormality, judges that the air-fuel ratio sensor is normal when the first speed of change of air-fuel ratio is faster than a speed of change used as reference for normality, and judges that a hold should be put on judgment when the first speed of change of air-fuel ratio is between the speed of change used as reference for abnormality and the speed of change used as reference for normality.