PURPOSE: To decrease pressure difference between a fuel system and an air system when loads and flow rate are varied and remove failure caused by pressure difference by installing a valve and a pressure detector and controlling the valve.

CONSTITUTION: A flow rate control valve 20 in an air system, and pressure detectors 21a and 21b measuring pressure of a fuel system and the air system are installed. A differential pressure signal and an aimed air flow rate signal from a function generator 18b are inputted to a computing element 22. The computing elements 22 perform operation which is specified in controllers 16b and 16c of valves 12 and 20 respectively and output the results. The valve 12 and 20 operate in accordance with open ratio signals of controllers 16b and 16c. The controller 16b is a PI controller, and the controller 16c is an I controller, and integrating time constant of 16b is larger than that of 16c. A fuel flow rate control system inputs load current I to a function generator 18a, and operates an aimed flow rate, and compares an operating results with a measuring value of a flow meter 19a, and controls a valve 8 by PI control. Time constant of a PI controller 16a is same as that of the PI controller 16b. The function generators 18a and 18b give linear aimed flow rate to current signal, and the operation results of 18b always give 2.5 times larger flow rate than the results of 18a.