PURPOSE: To form a fuel system as one system of simple constitution and to enable stable fuel supply at a proper air-fuel ratio.

CONSTITUTION: In a controlled pressure chamber 13 controlled to negative pressure at which air compression does not occur, air is mixed with fuel to obtain an air-fuel mixture of a constant air-fuel ratio. Thereafter, the air-fuel mixture and air measured by a main intake throttling part 8 are mixed at the main intake throttling part 8 or a main intake passage 3 located downstream of the main intake throttling part 8. Since the air-fuel mixture and the air are both compressed fluids, the air measured by the intake throttling part 8 is proportional to the air-fuel mixture injected from the controlled pressure chamber 13, and the air-fuel ratio of the air-fuel mixture produced inside the main intake passage 3 is constant over the whole rotation range. Therefore, fuel supply using one system is made possible.