An internal combustion engine comprises an exhaust purification catalyst arranged in an exhaust passage of the internal combustion engine; an upstream side air-fuel ratio sensor for detecting an air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst; and a downstream side air-fuel ratio sensor for detecting an air-fuel ratio of exhaust gas flowing out from the exhaust purification catalyst. An abnormality diagnosis system of air-fuel ratio sensors judges that at least one of the air-fuel ratio sensors has become abnormal when a difference or ratio between an output value of said upstream side air-fuel ratio sensor and an output value of said downstream side air-fuel ratio sensor becomes outside a predetermined range of normal difference or predetermined range of normal ratio during atmospheric gas introduction control where the exhaust gas flowing into the exhaust purification catalyst becomes atmospheric gas.