PURPOSE: To prevent the reduction in torque of a two-cycle engine and the like by controlling the quantity of corrective air bleed by use of imbalance of negative pressure generated accompanied with the opening/closing of a throttle valve thereby decreasing the quantity of bleed air mixed with main fuel in case of more than the medium quantity required by the engine.

CONSTITUTION: Negative pressure in a venturi 3 is low when a throttle valve 4 is opened small, and as negative pressure acting on respective ports 26a and 27a opened at the high and low positions of the narrowest section 3a which are located symmetrically, is identical, a control valve 15 is opened wide by means of a spring (25) action. Then the quantity of suction air will be medium as the opening of the throttle valve 4 is increased, as negative pressure acting on the port 26a at the high position becomes high, the difference in pressure is generated between two negative pressure chambers 21 and 23 for moving a diaphragm 22 to No.1 negative pressure chamber 21 so as to make the opening of the control valve 15 small. As a result, as the quantity of corrective air bleed passing through a passage 14 is decreased, main fuel sucked out of a main nozzle 8a is increased.