A glass ceramic sintered body containing gahnite and cordierite as crystal phases, having a thermal expansion coefficient at 40 to 400 C. of not larger than 510^-6/ C., a dielectric constant of not larger than 7 and a Young's modulus of not larger than 150 GPa. A wiring board having an insulating substrate made of the glass ceramic sintered body features very high reliability in the primary mounting and in the secondary mounting.