PURPOSE: To prevent adhesion of tar to the throttle valve by employing a cyclon structure at the gas intake section while arranging a mixture gas throttle valve having two coaxial valves at the lower section thereby centrifugally separating the tar component in the dry distillation gas at the gas intake port.

CONSTITUTION: The distillation gas G is led in as the fuel gas through a gas regulation valve 1 from above a mixer 2 to produce a mixture gas M with the air A led in through the top of the mixer 2. Here, a cyclon structure is employed at the gas intake section 2A while a mixture gas throttle valve 4 is arranged at the lower section of said cyclon section 2B while directing the lead-in port for the intake manifold 3 or the pressure drop side downward. The mixture gas throttle valve 4 is formed to have two valves 6A, 6B at the relative position on single supporting shaft 5 which is rotated through the lever 8 of the governor 7. Consequently, insufficient operation of mixture gas throttle valve 4 can be prevented.