To smoothly switch an idle operation to a rated operation without any deterioration in safety of a diesel engine operation.

In a switch from the idle operation to the rated operation, a rotation control mode of a diesel engine 2 is switched from an isochronous control mode X to a droop control mode Y, and at the same time, a target engine speed NE in the droop control mode is changed to a corrected target engine speed Nec higher than a target idle engine speed Neid. Subsequently, the target engine speed NE of the diesel engine 2 is raised from the corrected target engine speed Nec to a target rated engine speed Ner.