Abstract: A fuel supplement supplying device for an internal combustion engine to add a liquid additive such as water, methanol, or the like into the air being introduced prior to be mixed with a fuel is disclosed. The device includes a hollow vaporizing head of porous material disposed within the induction duct leading to the combustion chamber of the internal combustion engine to define a constricted annular zone between the head and the induction duct. The liquid additive as above is fed from a reservoir via a feed line to the vaporizing head where the same is subjected to the negative pressure in response to a vacuum developed in the induction duct upon operation of the engine to whereby be sucked or seeped out through the porous wall of the vaporizing head into the stream of the air passing through said constricted annular zone. The liquid additive thus seeped out is vaporized on the surface of the head in the passing air to moisten the introducing air prior to being mixed with fuel.