A spark plug is provided which is constructed to ensure a higher degree of air-tightness. The spark plug includes a cylindrical metal housing within which an insulation porcelain is disposed. The housing has a hexagon head formed on an outer wall thereof and an annular chamber defined between itself and the insulation porcelain within which a sealing powder is disposed. The housing has an annular extension formed on the hexagon head. The annular extension is crimped inward to compress the sealing powder to enhance the air-tightness between the housing and the insulation porcelain. The annular extension has featured dimensions in order to ensure a desired degree of the air-tightness.