PURPOSE: To improve acceleration respons property after decelerating operation by a method wherein an atmosphere releasing valve is opened when the output of an engine load detecting means has arrived at a predetermined value or higher to introduce the ambient air into the vacuum chamber of a fuel intercepting valve.

CONSTITUTION: The titled device is consisting of a vaporizer 3, vaporizing LPG fuel, and the fuel intercepting valve 11 as well as a flow amount regulating jet 12, interposed in a fuel path connecting said vaporizer and a mixer 13, supplying the vaporized fuel into the suction path 14. The fuel intercepting valve 11 intercepts the fuel path 10 by closing the valve body 26 by the suction vacuum introduced through the vacuum path 23 through a diaphragm 26 when the suction vacuum, introduced into the pressure chamber 17 of a vacuum control valve 16 arrives at a predetermined value or higher and the valve body 20 is opened. In this case, an atmosphere releasing valve 42 is provided under connecting it to the vacuum path 23. When a throttle valve 15 is opened to an opening degree higher than a predetermined opening degree and a load detecting switch 40 is made ON, said valve 42 is opened to open the fuel intercepting valve 11 immediately.