in the open condition the negative pressure actuator 22 does not operate and the air valve 12 is kept at the initial opening degree further the negative pressure of the depression chamber 18 is applied to the depression chamber 45 of the first switching valve 42 through the passage 46 but the first switching valve 42 also operates to maintain the main negative pressure passage 39 in the closed condition similarly the second switching valve 56 and the third switching valve 59 also maintain the branch passage 54b including the main jet 55b and the branch passage 57b including the main jet 58b in the close conditions respectively since the negative pressure produced downstream of the gap 15a of the air valve 12 is applied to the depression chamber 36 of the air section of regulator 34 through the confluence chamber 41a of the subsidiary negative pressure passage 40 and the negative pressure introduction passage 41 in the conditions described above the first diaphragm 35 and the connecting member 66 are displaced upward then the second diaphragm 51 is displaced also upward and the needle valve 66a opens the fuel injection port 43 whereby the fuel is ejected from the fuel injection chamber 53 into the suction tube 1 through the fuel injection port 53a and the fuel injection passage 65 accordingly the fuel pressure is lowered in the fuel injection chamber 53 and the fuel is supplied in a quantity corresponding to the ejection rate from the fuel pressure chamber 52 into the fuel injection chamber 53 through the fuel passage 54 and the pilot jet 55a further the fuel to be fed into the fuel injection chamber 53.