Abstract: When refreshing a catalyst (22) disposed in an exhaust line (20) during operation of a diesel engine (1), diesel engine control increases supercharging pressure by throttling nozzles of a turbocharger (25) so as to increase an amount of air that is charged into a combustion chamber (4) and then opens an exhaust gas recirculation valve (24) and holds it open until a exhaust gas recirculation ratio reaches a specified value. Subsequently, the diesel engine control increases an amount of fuel injection until a mean combustion chamber air-to-fuel ratio reaches a value near a stoichiometric air-to-fuel ratio so as to lower the oxygen concentration of exhaust gas, thereby causing the catalyst (22) to release NOx therefrom and purify the exhaust gas.