An object is to provide a diesel engine that eliminates discomfort when an operator of a working, machine coupled to the diesel engine is at work while a particulate filter is being regenerated. A diesel engine includes an exhaust emission purifier and selecting means capable of selecting any one control method from isochronous control and droop control. The exhaust emission purifier includes a particulate filter and a pressure regulation valve. The particulate filter is disposed along an exhaust stream to purify exhaust gas. The pressure regulation valve is configured to apply a load onto the diesel engine so as to raise a temperature of the particulate filter so that a particulate accumulable in the particulate filter is forcibly removed and that the particulate filter is regenerated. The selecting means is configured to select the isochronous control to control the diesel engine when the load is applied by the pressure regulation valve.