Fuel supply system (10) of engine (1) includes a fuel supply pump (11), a common rail (12), fuel injector valves (13), and a cut-off valve (14), and a reducing-agent supplying device which includes a metering valve (16), a fuel introducing nozzle (17) and a secondary fuel supply passage (P2) and which is arranged to introduce a fuel as a reducing agent into an exhaust system (40) of the engine under the control of an electronic engine control unit (80), which control the application of an electric current to the metering valve. The engine control unit constitutes a part of a diagnosing apparatus arranged to diagnose the reducing-agent supplying device, on the basis of a change of an air/fuel ratio (A/F) of an exhaust emission from the engine, which air/fuel ratio is continuously detected during the period of introduction of the fuel into the exhaust system.