A combustion device includes an ammonia supply unit supplying primary reduction ammonia as a nitrogen oxide reducing agent into a combustor and mixing secondary reduction ammonia with combustion exhaust gas discharged from the combustor to reduce nitrogen oxide contained in the combustion exhaust gas and a controller configured to control at least one of the amount of supply of the primary reduction ammonia and the amount of mixing of the secondary reduction ammonia with the combustion exhaust gas in accordance with concentrations of residual nitrogen oxide and residual ammonia contained in the combustion exhaust gas after being discharged from the combustor.