PURPOSE: To prevent overheat of a catalyst by arranging a power system in parallel to a main system, detecting opening of a throttle valve, and increasing a quantity of fuel so that the airfuel ratio exceeds the theoretical air-fuel ratio by actuating the power system when its opening exceeds a prescribed value.

CONSTITUTION: When opening TA of a throttle valve 24 detected by an opening sensor 7 becomes a prescribed value TAa or more in an operating condition in the theoretical air-fuel ratio, a control valve 5 is closed by a control part 6. Therefore, negative pressure introduced to a power piston 18 through negative pressure pipe lines 41 and 42 until then is cut off, and the power piston 18 is moved downward by a built-in spring, and a power valve 17 is pushed down. Then, fuel of a float chamber 11 joins to fuel deriving from a main jet 14 through the power valve 17 and a power jet 16, and a quantity is increased more than a fuel quantity in the theoretical air-fuel ratio, and as a result, a combustion temperature is lowered, and a rise in a catalyst temperature is restrained.