A rod-in-tube method for producing a glass preform for use in the fabrication of an optical fiber, which comprises steps of inserting a glass rod constituting a core material in a glass tube constituting a cladding material, heating the rod-tube composite by an outer heating source with introducing, in the gap between the rod and the tube, a gaseous mixture containing a silicon halogenide, a fluorine-containing compound and oxygen gas in which a ratio of silicon and fluorine (Si/F) is larger than 1/300 and smaller than 1/5, and heating and fusing the composite at a temperature not lower than 1,900.degree. C. with filling the gap by a gaseous mixture comprising a halogen-containing compound and oxygen gas; from which glass preform, an optical fiber with low attenuation of light transmission, particularly in a long wavelength range, is fabricated.