An apparatus for reducing nitrogen oxide in an exhaust pipe, comprising: a nitrogen oxide sensor that is mounted in the exhaust pipe and that measures an amount of the nitrogen oxide in exhaust gas; a nozzle that is disposed on the exhaust pipe to be spaced apart from the nitrogen oxide sensor to a downstream side of the nitrogen oxide sensor by a first predetermined length and that injects urea corresponding to the amount of the nitrogen oxide; a mixer that mixes the urea with the exhaust gas and that is disposed at a downstream side of the nozzle on the exhaust pipe to be spaced apart by a second predetermined length from the nozzle; a de-nitrification catalyst reaction device that is disposed at a downstream side of the mixer on the exhaust pipe, purifies the nitrogen oxide, and includes an inlet portion having a predetermined length; and a stabilizing portion disposed between the mixer and the inlet portion of the de-nitrification catalyst reaction device, wherein the stabilizing portion has a third predetermined length; wherein the third predetermined length ranges from approximately 1 to approximately 2 times longer than a diameter (d) of the exhaust pipe; wherein the first and second predetermined lengths (L1, L2) respectively range from approximately 1 to approximately 2 times longer than the diameter (d) of the exhaust pipe; wherein the stabilizing portion has a dual pipe structure formed of an inner pipe enclosed by an outer pipe; wherein an injection angle (α) of the nozzle ranges from approximately 15 to approximately 45 degrees with respect to a longitudinal axis of the exhaust pipe; wherein the predetermined length of the inlet portion of the de-nitrification catalyst reaction device ranges from approximately 0.7 to approximately 2 times longer than a diameter of the de-nitrification catalyst reaction device; wherein a portion of the exhaust pipe disposed between the nitrogen oxide sensor and the mixer has a uniform diameter; wherein a diameter of the stabilizing portion is substantially the same as that of the exhaust pipe; wherein the stabilizing portion is disposed coaxial with the exhaust pipe; wherein the first predetermined length ranges from approximately 1.5 to approximately 2 times longer than the diameter (d) of the exhaust pipe; wherein the inlet portion of the de-nitrification catalyst reaction device has the dual pipe structure formed of the inner pipe enclosed by the outer pipe; and wherein first ends of the inner pipe and the outer pipe in the inlet portion of the de-nitrification catalyst reaction device open to form an opening therebetween. a nitrogen oxide sensor that is mounted in the exhaust pipe and that measures an amount of the nitrogen oxide in exhaust gas; a nozzle that is disposed on the exhaust pipe to be spaced apart from the nitrogen oxide sensor to a downstream side of the nitrogen oxide sensor by a first predetermined length and that injects urea corresponding to the amount of the nitrogen oxide; a mixer that mixes the urea with the exhaust gas and that is disposed at a downstream side of the nozzle on the exhaust pipe to be spaced apart by a second predetermined length from the nozzle; a de-nitrification catalyst reaction device that is disposed at a downstream side of the mixer on the exhaust pipe, purifies the nitrogen oxide, and includes an inlet portion having a predetermined length; and a stabilizing portion disposed between the mixer and the inlet portion of the de-nitrification catalyst reaction device, wherein the stabilizing portion has a third predetermined length; wherein the third predetermined length ranges from approximately 1 to approximately 2 times longer than a diameter (d) of the exhaust pipe; wherein the first and second predetermined lengths (L1, L2) respectively range from approximately 1 to approximately 2 times longer than the diameter (d) of the exhaust pipe; wherein the stabilizing portion has a dual pipe structure formed of an inner pipe enclosed by an outer pipe; wherein an injection angle (α) of the nozzle ranges from approximately 15 to approximately 45 degrees with respect to a longitudinal axis of the exhaust pipe; wherein the predetermined length of the inlet portion of the de-nitrification catalyst reaction device ranges from approximately 0.7 to approximately 2 times longer than a diameter of the de-nitrification catalyst reaction device; wherein a portion of the exhaust pipe disposed between the nitrogen oxide sensor and the mixer has a uniform diameter; wherein a diameter of the stabilizing portion is substantially the same as that of the exhaust pipe; wherein the stabilizing portion is disposed coaxial with the exhaust pipe; wherein the first predetermined length ranges from approximately 1.5 to approximately 2 times longer than the diameter (d) of the exhaust pipe; wherein the inlet portion of the de-nitrification catalyst reaction device has the dual pipe structure formed of the inner pipe enclosed by the outer pipe; and wherein first ends of the inner pipe and the outer pipe in the inlet portion of the de-nitrification catalyst reaction device open to form an opening therebetween.