The following technique is known to synthesize C/C (carbon fiber-reinforced carbon) composites. The organic matter in the preformed yarn (plastic straw covered yarn including bundles of long carbon fibers, carbon powder, and organic binder) is pyrolyzed at 500 A degrees C and concurrently hot-pressed. Then, the carbon ingredient is graphitized in an atmosphere of nitrogen at 2000 A degrees C. The authors used the above mentioned C/C composites as a starting material and developed a dense Si-SiC matrix C/C composites in which most long carbon fibers remain without reacting with Si which is infiltrated in argon at 1600 A degrees C and 100 Pa. As a result, production of 1 x 2 m large size plates free from warps and cracks was attained in NGK Insulators, Ltd. This mechanism consists of three steps. First, a trunk-shaped Si-SiC matrix is synthesized between yarn and yarn. Then a trunk-shaped Si-SiC matrix extends a yarn by force. Only differential gap is made in a yarn surface. Finally, branch-shaped Si-SiC matrix is synthesized so that a trunk-shaped Si-SiC matrix leads to the yarn inside.