An engine system having an exhaust-pipe fuel injector, an oxidizing catalyst device, a NOx purification device and an apparatus for diagnosing causes of NOx conversion efficiency degradation, wherein the apparatus for diagnosing causes of NOx conversion efficiency degradation comprises: a diagnosis exhaust-pipe fuel injection control unit; a NO X purification device failure specifying unit; and an oxidizing catalyst device failure specifying unit; characterized in that the apparatus for diagnosing causes of NO X conversion efficiency degradation further comprises an engine-side failure specifying unit configured to specify a cause of NOx conversion efficiency degradation as an engine-side failure when a NOx conversion efficiency, which is calculated from a NOx concentration ratio, drops below a first threshold value, with the exhaust-pipe fuel injection being not yet performed by the exhaust-pipe fuel injector, and when a difference between an actual exhaust gas air-fuel ratio and a theoretical exhaust gas air-fuel ratio at a position downstream of the NOx purification device is greater than a second threshold value; in that the diagnosis exhaust-pipe fuel injection control unit is configured to cause the exhaust-pipe fuel injector to perform exhaust-pipe fuel injection for diagnosis purposes when the NO X conversion efficiency, which is calculated from the NOx concentration ratio, drops below the first threshold value, with the exhaust-pipe fuel injection being not yet performed by the exhaust-pipe fuel injector, and when the difference between the actual exhaust gas air-fuel ratio and the theoretical exhaust gas air-fuel ratio at the position downstream of the NOx purification device is equal to or smaller than the second threshold value; in that the NO X purification device failure specifying unit is configured to specify a cause of NO X conversion efficiency degradation as a NO X purification device failure if a difference between an actual heat generation amount and a theoretical heat generation amount in the oxidizing catalyst device at the time of the exhaust-pipe fuel injection for the diagnosis purposes is equal to or smaller than a third threshold value; and in that the oxidizing catalyst device failure specifying unit is configured to specify a cause of NO X conversion efficiency degradation as an oxidizing catalyst device failure if the difference between the actual heat generation amount and the theoretical heat generation amount in the oxidizing catalyst device at the time of the exhaust-pipe fuel injection for the diagnosis purposes exceeds the third threshold value and if a difference between an actual heat generation amount and a theoretical heat generation amount in the NOx purification device is equal to or smaller than a fourth threshold value. a diagnosis exhaust-pipe fuel injection control unit; a NO X purification device failure specifying unit; and an oxidizing catalyst device failure specifying unit; characterized in that the apparatus for diagnosing causes of NO X conversion efficiency degradation further comprises an engine-side failure specifying unit configured to specify a cause of NOx conversion efficiency degradation as an engine-side failure when a NOx conversion efficiency, which is calculated from a NOx concentration ratio, drops below a first threshold value, with the exhaust-pipe fuel injection being not yet performed by the exhaust-pipe fuel injector, and when a difference between an actual exhaust gas air-fuel ratio and a theoretical exhaust gas air-fuel ratio at a position downstream of the NOx purification device is greater than a second threshold value; in that the diagnosis exhaust-pipe fuel injection control unit is configured to cause the exhaust-pipe fuel injector to perform exhaust-pipe fuel injection for diagnosis purposes when the NO X conversion efficiency, which is calculated from the NOx concentration ratio, drops below the first threshold value, with the exhaust-pipe fuel injection being not yet performed by the exhaust-pipe fuel injector, and when the difference between the actual exhaust gas air-fuel ratio and the theoretical exhaust gas air-fuel ratio at the position downstream of the NOx purification device is equal to or smaller than the second threshold value; in that the NO X purification device failure specifying unit is configured to specify a cause of NO X conversion efficiency degradation as a NO X purification device failure if a difference between an actual heat generation amount and a theoretical heat generation amount in the oxidizing catalyst device at the time of the exhaust-pipe fuel injection for the diagnosis purposes is equal to or smaller than a third threshold value; and in that the oxidizing catalyst device failure specifying unit is configured to specify a cause of NO X conversion efficiency degradation as an oxidizing catalyst device failure if the difference between the actual heat generation amount and the theoretical heat generation amount in the oxidizing catalyst device at the time of the exhaust-pipe fuel injection for the diagnosis purposes exceeds the third threshold value and if a difference between an actual heat generation amount and a theoretical heat generation amount in the NOx purification device is equal to or smaller than a fourth threshold value.