Spark plug

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PROBLEM TO BE SOLVED : To provide an insulator for a spark plug capable of improving durability to a dielectric breakdown and being manufactured at a low cost. SOLUTION : The insulator for a spark plug 2 includes a first part 2p and a second part 2s, respectively formed of alumina ceramic mainly consisting of alumina. The first part 2p on a side of a spark discharge gap g, which is likely to create a problem, such as the dielectric breakdowns resulting from applying a high voltage is formed of the alumina ceramic with higher insulating performance (to be specific, having a high breakdown voltage) than the second part 2s, which is relatively not likely to create the problem, such as dielectric breakdowns. For example a content rate of the alumina in the first part 2p is higher than that of the second part 2s. Or the percent content of a rear earth element component of the alumina ceramic in the first part 2p is higher than that of the second part 2s. Also the percent content of an alkali metal component of the alumina ceramic in the first part 2p is lower than that of the second part 2s.