A denitration device and a denitration method in which denitration is performed efficiently and in a stable manner in a lower-reaction-temperature region without using a catalyst. The denitration device is provided with a combustion chamber, a denitration agent feed means for feeding a denitration agent into the combustion chamber, an exhaust pipe, and an OH-radical-generating substance feed means for feeding an OH radical-generating substance into the exhaust pipe. The denitration agent feed means feeds a denitration agent into the exhaust gas of the combustion chamber to perform a first denitration reaction step, and the OH-radical-generating substance feed means feeds the OH-radical-generating substance into the exhaust gas in the exhaust pipe to perform a second denitration reaction step.