Manufacturing method of glass base material for optical fiber

A manufacturing method of glass base material for optical fiber performing:

• preparation of a porous base material for optical fiber in which a porous glass layer is formed around the periphery of a core rod;
• sintering preparation, in which the porous base material for optical fiber is hung in a furnace core tube of a sintering apparatus;
• and
• sintering in which a mixture of halogen-containing gas and inert gas is constantly flowing into the furnace core tube, and the porous base material for optical fiber is made into transparent glass by heating the porous base material for optical fiber while moving a heater relative to the porous base material for optical fiber from one end of the core rod to an other end, to obtain the glass base material for optical fiber,
• wherein, in the process of moving the heater relative to the porous base material for optical fiber in the sintering, the concentration of halogen-containing gas in the mixed gas is reduced, and
• wherein a concentration of the halogen-containing gas at a start of an execution of the sintering is C0 and a concentration of the halogen-containing gas at a time when the other end of the core rod passes a vertical midpoint of the heater is C1, and a relationship between C0 and C1 is 0.25×C0≤C1≤0.5×C0.