the silicon oxide represented by the formula sio x 1 u2266 x lt 1 2 is produced by a method known in the art for example by heating a silicon dioxide based oxide powder such as fumed silica and colloidal silica with a substance which reduces such oxide powder for example a powder mixture of a metal silicon and carbon in an inert gas or under reduced pressure to a temperature in the range of 1100 to 1600 u00b0 c for generation of silicon monoxide gas allowing this silicon monoxide gas to precipitate on the surface of the substrate and pulverizing the precipitated to a predetermined grain size the thus obtained silicon oxide powder is not particularly limited for its physical properties however this silicon oxide powder preferably has an average particle diameter of 0 3 to 100 u03bcm and in particular 0 5 to 80 u03bcm use of the silicon oxide powder having an average particle diameter below such range may result in an extreme increase in the cost of pulverization in relation to the only slight increase in the strength of the sintered body on the contrary an average particle diameter exceeding such range results in the decrease in the area of contact between the particles and the sintered body using such powder may suffer from insufficient strength it is to be noted that the average particle diameter is the value measured in terms of a weight average or a median diameter in the.