In a method for manufacturing an optical fiber, a dummy rod is connected to a vicinity of an upper end of a glass preform which includes a diameter-reduced portion at an upper portion thereof, and the glass preform is heated to be melt in a drawing furnace to draw an optical fiber. A cap member is provided at a position of the diameter-reduced portion, and an upper end of the cap member is disposed close to a lower end of the dummy rod. When an orthogonal coordinate axis is set, in which an outer diameter ratio of the cap member to the glass preform is set as a horizontal axis x, and an outer diameter ratio of the dummy rod to the glass preform is set as a vertical axis y, the x and the y satisfy an expression y≥0.1527×x−3.103.