Silicon oxide containing active silicon represented by the general formula: SiOx wherein x is from 0.8 to 1.9, when analyzed by solid state NMR (29Si DD/MAS) with a sufficient relaxation time set, exhibits a spectrum having two separate peaks, a broad peak (A1) centered at -70 ppm and a broad peak (A2) centered at -110 ppm, the area ratio A1/A2 being in the range of 0.1.ltoreq.A1/A2.ltoreq.1.0.