PURPOSE: To enable the fuel flow rate being supplied through a main passage to be easily controlled in accordance with the composition of the fuel to be used.

CONSTITUTION: In the vicinity of a driver's seat, a variable volume controller 10 for controlling flow rate that can be operated so as to be set at the graduation indicating the composition of LPG fuel is installed, and the displacement of the knob 11 of this variable volume controller 10 is converted into an electric signal, and this is input into a controller 13. The device is constituted in such a way that the controller 13 determines the displacement of a stepping motor 14 from a predetermined map on the basis of the input electric signal, and outputs an operation command to the stepping motor 14, and on the basis of this command, the stepping motor 14 turns a power-adjusting screw 9 in the main passage 8 of a LPG carburetor 6 for controlling the cross-sectional area of the passage.