PURPOSE: To prevent the output lowering of an engine by making an opening/ closing valve provided at a sub fuel regulator conduct the action of opening by means of negative pressure in a connection flow passage that is generated in a case in which the supply quantity of main fuel supplied from a main fuel source becomes insufficient in regard to an engine demand quantity.

CONSTITUTION: A main fuel source to supply low-pollution gas fuel as main fuel is connected to the carburetor 2 of an engine 1 through a main fuel regulator 4. A sub fuel source to supply sub fuel whose calorie is higher than that of the main fuel, is connected through a sub fuel regulator 11 to the halfway portion of a connection flow passage 5 between the main fuel regulator 4 and the carburetor 2. An opening/closing valve provided at the sub fuel regulator 11 is made to conduct the action of opening by means of negative pressure in a connection flow passage 12 that is generated in a case in which the supply quantity of the main fuel supplied from the main fuel source becomes insufficient in regard to an engine demand quantity. As a result, the output performance of the engine 1 can be kept satisfactory and efficient, and the preservation of atmosphere can be realized.