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. <EOS>