PURPOSE: To quicken response by limiting engine fuel supply to one of a plurality of target quantities being set, in step, according to the difference between the rated of non-utility generator and the set value of generator output obtained through speed control at that time.

CONSTITUTION: A non-utility generator 2 normally performs speed control based on a signal fed from a receiving power transducer 10. When the receiving power drops to a level slightly higher than the reverse power flow starting level, generator output set value WGs obtained under speed control at that time and generator output WG being subjected to speed control are locked then fuel supply to an engine 3 is limited based on a plurality of set values being set according to the difference between the rated value of the generator 2 and WGs. If 95%>WGs≥75%, for example, valve is closed to supply 50% fuel thus bringing the output from the generator 2 to 0.5WCs. It the generator output is lower than 0.45WGs, when receiving power is recovered, control conditions are reset so that fuel supply limit for the engine 3 are released.