An abnormality diagnosis apparatus for a NOx sensor, comprising: a selective reduction NOx catalyst provided in an exhaust passage of an internal-combustion engine; reducing agent addition means for selectively adding reducing agent to the NOx catalyst; before-catalyst NOx concentration acquisition means for estimating a before NOx concentration (Ce) of exhaust gas at an upstream side of the NOx catalyst; an after-catalyst NOx sensor for detecting an after NOx concentration (Cr) of exhaust gas at a downstream side of the NOx catalyst; characterized by comprising: abnormality determination means for determining an abnormality of the after-catalyst NOx sensor, during the stoppage of an addition of reducing agent by the reducing agent addition means, stoppage request signal generating means for generating a stoppage request signal for stopping the internal-combustion engine; stoppage delay means for delaying, when the stoppage request signal is generated by the stoppage request signal generating means, the stoppage of the internal-combustion engine by a predetermined delay time; and forced idling control means for executing a forced idling control for causing the internal-combustion engine to perform an idling operation during the delay time; wherein the reducing agent addition means are configured to stop an addition of reducing agent during the delay time, and wherein the abnormality determination means are configured to determine that the after-catalyst NOx sensor is abnormal when a difference (ΔC) between the after NOx concentration (Cr) detected by the after-catalyst NOx sensor and the before NOx concentration (Ce) estimated by the before-catalyst NOx concentration acquisition means is higher than a predetermined value (ΔCs). a selective reduction NOx catalyst provided in an exhaust passage of an internal-combustion engine; reducing agent addition means for selectively adding reducing agent to the NOx catalyst; before-catalyst NOx concentration acquisition means for estimating a before NOx concentration (Ce) of exhaust gas at an upstream side of the NOx catalyst; abnormality determination means for determining an abnormality of the after-catalyst NOx sensor, during the stoppage of an addition of reducing agent by the reducing agent addition means, stoppage request signal generating means for generating a stoppage request signal for stopping the internal-combustion engine; stoppage delay means for delaying, when the stoppage request signal is generated by the stoppage request signal generating means, the stoppage of the internal-combustion engine by a predetermined delay time; and forced idling control means for executing a forced idling control for causing the internal-combustion engine to perform an idling operation during the delay time; wherein the reducing agent addition means are configured to stop an addition of reducing agent during the delay time, and wherein the abnormality determination means are configured to determine that the after-catalyst NOx sensor is abnormal when a difference (ΔC) between the after NOx concentration (Cr) detected by the after-catalyst NOx sensor and the before NOx concentration (Ce) estimated by the before-catalyst NOx concentration acquisition means is higher than a predetermined value (ΔCs).