A method for producing an optical fiber, more particularly a process for producing a preform in the course of the production of the optical fiber, which comprises the steps of using converging light beams as a heating source, heating the end of a starting rod with such light beams and blowing the fiber material-producing gases to the heated portion of the starting rod to grow the preform. The invention also provides a process for producing a low-loss optical fiber wherein the attenuation of light is greatly improved by growing the preform under reduced pressure or by applying supersonic waves to the heated end of the starting rod. There is also provided a process for producing a grated index optical fiber wherein the preform is produced by providing a specific distribution character to the converging light beams and to the impurity concentration of the blown gas so that the impurity concentration distribution will differ along the direction perpendicular to the preform growing direction.