An object of the present invention is to restrain the emission amount of unburned fuel components while restraining the consumption of fuel with relatively high ignitability in a multi-fuel internal combustion engine. The present invention resides in such a fuel supply control system for a multi-fuel internal combustion engine that a first fuel with relatively high ignitability and a second fuel with relatively low ignitability are used as fuels and the first fuel is used as an ignition source to mixedly combust the first fuel and the second fuel, wherein a supply amount Qd of the first fuel is adjusted on the basis of a difference ΔRH between a first heat generation rate RHd which is a heat generation rate given when assuming that only the first fuel is combusted and an actual heat generation rate RHa which is a heat generation rate given when the fuels are actually combusted (S105 to 107, S109).