To provide a method for selectively reducing NOx contained in an exhaust gas catalytically at temperatures of 200-600°C.

A spray of liquid droplets containing a reducing agent which can reduce NOx and/or its precursor is injected into a duct through a spraying nozzle, and an exhaust gas loaded with the liquid droplets is run through a space formed between the plates of stacks of parallel plates in a gas duct. In this case, the plates of the first stack are arranged at an angle of about 45°C with the wall face of the gas duct and the plates of the second stack are arranged at about 90°C with the plates of the first stack and at about 45° with the wall face of the duct. The plates of the third stack are in parallel with the plates of the first stack. Thus the gas passes through the stacks flowing in a zigzag fashion. Further, the gas is run through an integrally formed catalyst having a parallel passageway on the wall face of the duct, so that NOx is selectively reduced with the reducing agent after the last stack.