Abstract: An organic silicon polymer is infiltrated and charged into gaps in a matrix phase of a formed fiber fabric, and its airtightness is increased by (a) CVI infiltration process 2 for forming the SIC matrix phase on the surface of the fiber fabric formed, (b) pressurized infiltrations process 4 for pressurizing the organic silicon polymer in the direction operating pressure is applied to the fiber fabric during use and infiltrating the organic silicon polymer into gaps in the aforementioned matrix phase, and (c) heating process 5 for heating the infiltrated fiber fabric at a high temperature. Thus, airtightness can be increased quickly, and fired work can be applied practically even to thrust chambers etc.