Sintering apparatus and method for sintering

A porous glass base material sintering method for housing and sintering the porous glass base material in a furnace core tube, comprising:

• heating the porous glass base material housed in the furnace core tube by heaters which surround the furnace core tube while maintaining a pressure of gas inside the furnace core tube higher than an atmospheric pressure;
• discharging, into a sealing chamber provided at a lid member covering an insertion hole, a part of the gas inside of the furnace core tube, through a space between a holding rod coupled with the porous glass base material and the lid member mounted at one end of the furnace core tube with the insertion hole through which the holding rod is inserted, and through a space between the holding rod and a cylindrical member through which the hold rod is inserted;
• discharging a part of the gas inside of the furnace core tube discharged to the sealing chamber from a discharging port provided at the sealing chamber to outside along with seal gas supplied from a supply port provided at the sealing chamber.