To provide a denitration method at low temperature in which high denitration performance is exhibited even at low temperature.

Exhaust gas generated in a boiler 1 is cooled by a heat exchanger 2, and introduced into a denitration reaction device 3, O3 is injected from an ozonizer 6 and NO contained in exhaust gas is oxidized to NO2. Injection of NH3 is performed from an ammonia injection device 8, and NOx contained in gas is reduced by a denitration catalyst 4, and discharged from a chimney 5 to the outside of the system. On one side, i-PrOH used as a reduction auxiliary is allowed to accompany NH3 to be injected from the injection device 8. The concentration of O3 is desirably regulated to 0.5-10 times of the concentration of NO.