A NOx sensor abnormality detection device senses a NOx concentration and an O2 concentration from output signals of a NOx sensor and calculates an estimated NOx concentration from the O2 concentration when an engine is in a steady operation state, when a zero point of the NOx concentration is sensed, and when an O2 sensing section of the NOx sensor is normal. The NOx sensor abnormality detection device calculates a sensed inclination and an estimated inclination of a NOx concentration output characteristic of the NOx sensor based on the sensed NOx concentration, the sensed O2 concentration and the estimated NOx concentration calculated from the O2 concentration. The NOx sensor abnormality detection device determines that the NOx sensor is faulty when an absolute value of a difference between the sensed inclination and the estimated inclination is larger than a predetermined value.