Abstract: It is an object of this invention to provide a gas fuel supply system in which a controllable range in the fuel injection quantity control by an injector is enlarged. According to one preferable aspect of this invention, the gas fuel supply system comprises an injector provided in an intake pipe of an internal combustion engine, a fuel supply passage for supplying the gas fuel to the injector, and means for controlling pressure of the gas fuel supplied to the injector through the fuel supply passage by using negative pressure in the intake pipe such that the pressure of the gas fuel increases if the negative pressure in the intake pipe decreases, and the pressure of the gas fuel decreases if the negative pressure in the intake pipe increases.