PURPOSE: To improve the controllability of the automatic load control by providing a shaft output meter on a propeller shaft and by using this signal as a feedback signal to a control unit correcting the pitch of a variable pitch propeller.

CONSTITUTION: The number of revolutions of a main engine is converted by a function generator 5' into an output signal corresponding to the number of revolutions and is used as an output preset signal, and the signal of a shaft output meter 16 provided on the variable pitch propeller side is fed back to perform the automatic load control. Therefore, even if the calorific value and the specific gravity, etc. of the fuel are different from the design values, no influence is affected, thus the controllability can be improved. When the main engine is being operated at the same number of revolutions an excess power is available, and when the shaft generator/motor 13 is in operation as a motor the feedback signal becomes larger than the preset value, thereby the propeller pitch correction signal corresponding to the difference is output.