Insulator for spark plug and spark plug

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PROBLEM TO BE SOLVED : To provide an insulating material having high withstand-voltage and high thermal shock resistance, and a spark plug equipped with the insulating material. SOLUTION : The insulating material for a spark plug is obtained by the process which comprises preparing the slurry containing alumina powder having its central particle diameter of 0.4 μm, powdered oxide of a rare earth element and SiO2 powder, B2O3 powder, CaO powder, BaO powder, MgO powder or the like at a prescribed ratio, forming the granule which is obtained by granulating the slurry, cutting and processing the obtained formed material, and burning the material at 1500-1650 deg.C for 1-8 hour(s) in the atmosphere. The obtained insulating material contains aluminum at a rate of 95 mass% or more in terms of oxide to the total sum of the components at 100 mass% and the rare earth element and Si at a rate (RRE/RSi) of 0.1-1.0 in terms of oxide. Further, the number of the particles, whose maximum length existing around the section 1 mm2 is more than 10 μm and aspect ratio is more than 3, is less than 10.