PURPOSE: To prevent dilution of air-fuel mixture during primary and secondary interlocking operation, by installing to the downstream of a throttle valve a fuel increasing valve which detects a pressure generated at the start of openig of the throttle valve of the double mixer of a liquefied gas internal combustion engine and feeds the stored fuel to a secondary venturi as an increases in the fuel supply.

CONSTITUTION: A pressure detecting chamber 60 is formed in a fuel increasing valve 6, and the pressure detecting chamber 60 is communicated with the downstream of a secondary throttle valve of a mixer 4 through a negative pressure passage 61. A fuel passage chamber 63 is formed in a manner to be separated away from the pressure chamber 60 by a diaphragm 62, the passage chamber 63 being communicated with the upstream of a secondary jet. A valve 66 is attached to a pressure responsing member 62 to open and close a passage leading therefrom to a fuel storing chamber 67 formed adjacent to the fuel passage chamber 63. The fuel storing chamber 67 is communicated with the primary chamber of a vaporizer 3.