A combustion device comprising: a plurality of burners that are provided in a wall (111,112) of a furnace disposed along a vertical direction and that are configured to supply fuel, primary air and burner secondary air into the furnace; a plurality of air supply ports that are provided on a vertical-direction upper side of the plurality of burners in the wall (111,112) of the furnace and that are configured to supply air supply port secondary air into the furnace; a plurality of secondary air dampers that are configured to adjust flow rates of the air supply port secondary air; a measurement unit that is configured to measure gas concentration in a measurement area on a horizontal section of a furnace outlet portion positioned on a vertical-direction upper side of the plurality of air supply ports in the furnace; and a control device that has a circuit configured to output flow rate control command values for adjusting openings of the plurality of secondary air dampers respectively; wherein: the plurality of air supply ports are divided into two or more groups; the measurement area is divided in advance into a plurality of divided areas associated with the divided groups correspondingly and respectively; and the control device is configured to calculate deviations between an average value of a gas concentration in the whole measurement area and values of gas concentrations in the respective divided areas of the measurement area, and to output, to the plurality of secondary air dampers respectively, the flow rate control command values for the air supply port secondary air to be supplied to the air supply ports belonging to the groups associated with the divided areas correspondingly and respectively, such that residual unburnt combustible content is suppressed, wherein the output is based on values each obtained by adding, to a corresponding one of the calculated deviations, a positive or negative bias value calculated based on a load of the combustion device and patterns of used burners of the plurality of burners. a plurality of burners that are provided in a wall (111,112) of a furnace disposed along a vertical direction and that are configured to supply fuel, primary air and burner secondary air into the furnace; a plurality of air supply ports that are provided on a vertical-direction upper side of the plurality of burners in the wall (111,112) of the furnace and that are configured to supply air supply port secondary air into the furnace; a plurality of secondary air dampers that are configured to adjust flow rates of the air supply port secondary air; a measurement unit that is configured to measure gas concentration in a measurement area on a horizontal section of a furnace outlet portion positioned on a vertical-direction upper side of the plurality of air supply ports in the furnace; and a control device that has a circuit configured to output flow rate control command values for adjusting openings of the plurality of secondary air dampers respectively; wherein: the plurality of air supply ports are divided into two or more groups; the measurement area is divided in advance into a plurality of divided areas associated with the divided groups correspondingly and respectively; and the control device is configured to calculate deviations between an average value of a gas concentration in the whole measurement area and values of gas concentrations in the respective divided areas of the measurement area, and to output, to the plurality of secondary air dampers respectively, the flow rate control command values for the air supply port secondary air to be supplied to the air supply ports belonging to the groups associated with the divided areas correspondingly and respectively, such that residual unburnt combustible content is suppressed, wherein the output is based on values each obtained by adding, to a corresponding one of the calculated deviations, a positive or negative bias value calculated based on a load of the combustion device and patterns of used burners of the plurality of burners. the plurality of air supply ports are divided into two or more groups; the measurement area is divided in advance into a plurality of divided areas associated with the divided groups correspondingly and respectively; and the control device is configured to calculate deviations between an average value of a gas concentration in the whole measurement area and values of gas concentrations in the respective divided areas of the measurement area, and to output, to the plurality of secondary air dampers respectively, the flow rate control command values for the air supply port secondary air to be supplied to the air supply ports belonging to the groups associated with the divided areas correspondingly and respectively, such that residual unburnt combustible content is suppressed, wherein the output is based on values each obtained by adding, to a corresponding one of the calculated deviations, a positive or negative bias value calculated based on a load of the combustion device and patterns of used burners of the plurality of burners.