Fiber manufacturing method

Provided is an optical fiber manufacturing method capable of surely suppressing an increase in hydrogen loss while suppressing the amount of deuterium used by using a simple apparatus. A method for manufacturing an optical fiber, wherein an optical fiber (3) having a core portion formed by adding germanium to pure silica glass and a cladding portion formed on the outer periphery of the core portion and made of pure silica glass is placed in a processing tank (12) filled with a gas containing deuterium at a predetermined concentration under a predetermined pressure, and the optical fiber (3) isexposed to the gas at a predetermined temperature for a predetermined time to diffuse deuterium molecules in the optical fiber (3), wherein at least one of the pressure in the processing tank (12), the deuterium concentration in the processing tank (12), the exposure temperature of the gas, and the exposure time of the gas is changed in accordance with the relative refractive index difference between the core portion and