A control device for a diesel engine, comprising: - a turbo charger that coaxially joins an intake compressor and an exhaust turbine, and that is configured to drive the intake compressor with energy of exhaust gas which flows into the exhaust turbine to pressurize intake air; - a fresh air/secondary air supply device (81; 82) configured to supply fresh air or secondary air to an exhaust passage upstream of the exhaust turbine; and - a controller that is programmed to - determine that engine operation conditions are in a gas flow stagnation region in which an engine torque does not increase even when a fuel injection amount is increased when an oxygen concentration of the exhaust gas or an excess air ratio of gas within the cylinders is less than a predetermined concentration or predetermined ratio indicating that the oxygen of the gas within the cylinders becomes insufficient and the fuel does not combust completely, a supercharging pressure of the compressor is higher than a pressure of the exhaust gas upstream of the exhaust turbine, and a temperature of the exhaust gas upstream of the exhaust turbine is higher than a predetermined temperature at which after-burning of unburned fuel in an exhaust gas reliably occurs, and - supply fresh air or secondary air to the exhaust passage upstream of the exhaust turbine by the fresh air/secondary air supply device (81; 82) when the engine operation conditions are in the gas flow stagnation region. - a turbo charger that coaxially joins an intake compressor and an exhaust turbine, and that is configured to drive the intake compressor with energy of exhaust gas which flows into the exhaust turbine to pressurize intake air; - a fresh air/secondary air supply device (81; 82) configured to supply fresh air or secondary air to an exhaust passage upstream of the exhaust turbine; and - a controller that is programmed to - determine that engine operation conditions are in a gas flow stagnation region in which an engine torque does not increase even when a fuel injection amount is increased when an oxygen concentration of the exhaust gas or an excess air ratio of gas within the cylinders is less than a predetermined concentration or predetermined ratio indicating that the oxygen of the gas within the cylinders becomes insufficient and the fuel does not combust completely, a supercharging pressure of the compressor is higher than a pressure of the exhaust gas upstream of the exhaust turbine, and a temperature of the exhaust gas upstream of the exhaust turbine is higher than a predetermined temperature at which after-burning of unburned fuel in an exhaust gas reliably occurs, and - supply fresh air or secondary air to the exhaust passage upstream of the exhaust turbine by the fresh air/secondary air supply device (81; 82) when the engine operation conditions are in the gas flow stagnation region. - determine that engine operation conditions are in a gas flow stagnation region in which an engine torque does not increase even when a fuel injection amount is increased when an oxygen concentration of the exhaust gas or an excess air ratio of gas within the cylinders is less than a predetermined concentration or predetermined ratio indicating that the oxygen of the gas within the cylinders becomes insufficient and the fuel does not combust completely, a supercharging pressure of the compressor is higher than a pressure of the exhaust gas upstream of the exhaust turbine, and a temperature of the exhaust gas upstream of the exhaust turbine is higher than a predetermined temperature at which after-burning of unburned fuel in an exhaust gas reliably occurs, and - supply fresh air or secondary air to the exhaust passage upstream of the exhaust turbine by the fresh air/secondary air supply device (81; 82) when the engine operation conditions are in the gas flow stagnation region.