Method and device for detecting oxygen concentration

An electronic control unit (90) for managing and controlling an operational state of a diesel engine (1) first opens a throttle valve (32) completely during the implementation of fuel cut so as to replace gas remaining in an exhaust system (40) with fresh air in an intake system (30), then closes the throttle valve (32) to stabilize a pressure and a temperature in the exhaust system (40), and makes atmospheric correction. The electronic control unit (90) performs a control of prohibiting fuel from being added within a predetermined period after the start of fuel cut.