PURPOSE: To obtain performance of smooth operation of an engine by preventing mixed gas from temporarily turning lean during its transition from the first side carburetor to the second side one.

CONSTITUTION: When the first side throttle valve 4 reaches just this side of the vicinity of full opening, one corner part of a pre-kick cam 12 somewhat pushes up a pin 16, and a medium lever 14 is swiveled a little to the right against elastic force of an intermediate spring 11, and a secondary side throttle lever 17 is also swiveled a little to the right, then a secondary side throttle valve 9 is opened to some extent. Subsequently, when the first side throttle valve 4 reaches the vicinity of full opening, a bent part 13a gets in touch with one end of an intermediate lever 10, which is swiveled to the left against the spring 11, while the other end of said lever 10 is separated from a pin 15. Then, the intermediate lever 14 is released from elastic force of the spring 11 and becomes freely rotatable, further the secondary side throttle lever 17 also turns freely, while the secondary side throttle valve 9 can be freely opened or closed by means of a opening forcing cam. Accordingly, even if the throttle valve 9 is suddenly opened, increase in quantity of fuel can well follow the increase of air.