A method of manufacturing an optical fiber in which the deviation of a cutoff wavelength from a design value can be minimized. It is found that, upon diameter reduction of a porous core glass body by heat treatment, when the diameter reduction ratio is set to a value larger than 0.90, a "spike" in the refractive index distribution curve of a core can be prevented, and the deviation of the cutoff wavelength from the design value can be minimized. Therefore, there is provided a method comprising the first step of depositing fine SiO.sub.2 particles and fine GeO.sub.2 particles, which are produced by a core formation burner, to grow a porous core glass body from a distal end of a rotating starting rod in an axial direction, the second step of performing diameter reduction by heating the grown porous core glass body by heating means while growing the porous core glass body, and the third step of depositing fine SiO.sub.2 particles produced by a cladding formation burner to form a porous cladding glass layer on an outer surface of the diameter-reduced porous core glass body while performing growing and diameter reduction of the porous core glass body, wherein an outer diameter of the porous core glass body after the third step of depositing is larger than 0.90 times an outer diameter before diameter reduction.