An abnormality diagnosis system of an air-fuel ratio sensor (40, 41) provided in an internal combustion engine, wherein the internal combustion engine has an intake passage in which a throttle valve is arranged and which leads an air-fuel mixture containing air and fuel to a combustion chamber, an exhaust passage discharging exhaust gas produced by combustion of the air-fuel mixture in the combustion chamber, and a blowby gas passage returning blowby gas in a crankcase to the downstream side of the throttle valve in the intake passage, and the air-fuel ratio sensor (40, 41) is provided in the exhaust passage and detects an air-fuel ratio of the exhaust gas flowing through the exhaust passage, the abnormality diagnosis system being characterized in that the abnormality diagnosis system is configured to acquire a blowby gas flow ratio showing a ratio of the flow of blowby gas to the flow of gas to the combustion chamber and an output current of the air-fuel ratio sensor (40, 41) during fuel cut control in which the internal combustion engine stops the feed of fuel to the combustion chamber and at a plurality of points of time of different flows of blowby gas passing through the blowby gas passage and flowing to the downstream side of the throttle valve in the intake passage, calculate an output current of the air-fuel ratio sensor (40, 41) corresponding to a blowby gas flow ratio smaller than the blowby gas flow ratios acquired at the plurality of points of time, based on the blowby gas flow ratios and output currents acquired at the plurality of points of time, and judge the air-fuel ratio sensor (40, 41) for abnormality based on the calculated output current. the internal combustion engine has an intake passage in which a throttle valve is arranged and which leads an air-fuel mixture containing air and fuel to a combustion chamber, an exhaust passage discharging exhaust gas produced by combustion of the air-fuel mixture in the combustion chamber, and a blowby gas passage returning blowby gas in a crankcase to the downstream side of the throttle valve in the intake passage, and the air-fuel ratio sensor (40, 41) is provided in the exhaust passage and detects an air-fuel ratio of the exhaust gas flowing through the exhaust passage, the abnormality diagnosis system being characterized in that the abnormality diagnosis system is configured to acquire a blowby gas flow ratio showing a ratio of the flow of blowby gas to the flow of gas to the combustion chamber and an output current of the air-fuel ratio sensor (40, 41) during fuel cut control in which the internal combustion engine stops the feed of fuel to the combustion chamber and at a plurality of points of time of different flows of blowby gas passing through the blowby gas passage and flowing to the downstream side of the throttle valve in the intake passage, calculate an output current of the air-fuel ratio sensor (40, 41) corresponding to a blowby gas flow ratio smaller than the blowby gas flow ratios acquired at the plurality of points of time, based on the blowby gas flow ratios and output currents acquired at the plurality of points of time, and judge the air-fuel ratio sensor (40, 41) for abnormality based on the calculated output current.