A combustion device of a gas turbine comprising: a combustor, an ammonia supply unit supplying primary reduction ammonia as a nitrogen oxide reducing agent into the 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 characterized in that the controller is configured to reduce the amount of mixing of the secondary reduction ammonia with the combustion exhaust gas in a case where the residual nitrogen oxide has a concentration equal to or less than a first regulatory value and the residual ammonia has a concentration exceeding a second regulatory value, and wherein the controller is configured to determine that the secondary reduction ammonia supply amount can be reduced in a case where the controller determines that the flow rate of the secondary reduction ammonia that is reduced falls within the supply range for denitrification even after the flow rate of the secondary reduction ammonia is reduced, the controller is configured to determine that the secondary reduction ammonia supply amount cannot be reduced in a case where the controller determines that the flow rate of the secondary reduction ammonia that is reduced falls out of the supply range for denitrification by the flow rate of the secondary reduction ammonia being reduced, the controller is configured to determine that the primary reduction ammonia supply amount can be increased in a case where the controller determines that the flow rate of the primary reduction ammonia that is increased falls within the supply range for reduction even after the flow rate of the primary reduction ammonia is increased, the controller is configured to determine that the primary reduction ammonia supply amount cannot be increased in a case where the controller determines that the flow rate of the primary reduction ammonia that is increased falls out of the supply range for reduction by the flow rate of the primary reduction ammonia being increased. a combustor, an ammonia supply unit supplying primary reduction ammonia as a nitrogen oxide reducing agent into the 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 characterized in that the controller is configured to reduce the amount of mixing of the secondary reduction ammonia with the combustion exhaust gas in a case where the residual nitrogen oxide has a concentration equal to or less than a first regulatory value and the residual ammonia has a concentration exceeding a second regulatory value, and wherein the controller is configured to determine that the secondary reduction ammonia supply amount can be reduced in a case where the controller determines that the flow rate of the secondary reduction ammonia that is reduced falls within the supply range for denitrification even after the flow rate of the secondary reduction ammonia is reduced, the controller is configured to determine that the secondary reduction ammonia supply amount cannot be reduced in a case where the controller determines that the flow rate of the secondary reduction ammonia that is reduced falls out of the supply range for denitrification by the flow rate of the secondary reduction ammonia being reduced, the controller is configured to determine that the primary reduction ammonia supply amount can be increased in a case where the controller determines that the flow rate of the primary reduction ammonia that is increased falls within the supply range for reduction even after the flow rate of the primary reduction ammonia is increased, the controller is configured to determine that the primary reduction ammonia supply amount cannot be increased in a case where the controller determines that the flow rate of the primary reduction ammonia that is increased falls out of the supply range for reduction by the flow rate of the primary reduction ammonia being increased.