Method for producing fluorine-containing silica glass

A method for producing fluorine-containing silica glass, comprising:

• decompression degassing which includes degassing an inside of a furnace core tube under reduced pressure while heating the inside of the furnace core tube, after inserting a porous silica glass body into the furnace core tube provided in an airtight container;
• fluorine adding which includes supplying a fluorine compound gas into the furnace core tube, exhausting the fluorine compound gas remaining in the furnace core tube to an outside of the furnace core tube, and first heat-treating the porous silica glass body, under reduced pressure;
• and
• second heat-treating the porous silica glass body under reduced pressure at a temperature higher than temperatures in the decompression degassing process and the fluorine adding process, wherein
• the decompression degassing, the fluorine adding, and the second heat-treating are performed within the same furnace core tube,
• in the fluorine adding, the first heat-treating is performed after stopping exhaustion of the furnace core tube,
• the decompression degassing process is performed at a temperature of 1100° C. or more and 1200° C. or less,
• the decompression degassing process has a heating time of at least 30 minutes at the temperature, and
• an ultimate pressure at an end of the decompression degassing process is less than 500 pascals.