The fluorine doping process of pure silica "soot" with various type of fluorine gases in the vapour-phase axial deposition (VAD) sintering process was investigated in detail. This investigation showed that the doping level is proportional to the 1/4 power of the concentration of the fluorine gas, and the achievable reduction of relative refractive index is -0.75% with this process. The kinetic investigation clarified that the fluorine doping process consists of the doping and dissociation reactions; the former reaction obeys the 1/2 power of the SiF4 partial pressure and the latter the two power of fluorine content [SiO1.5F], where SiO1.5F represents a silicon tetrahedron consisting of one fluorine and three bridging oxygen atoms in glass. The fluorine content is proportional to the 1/4 power of the SiF4 partial pressure at an equilibrium where the doping reaction is in competition with the dissociation reaction.