A spark plug is disclosed as including a metal shell 10 formed with first and second bores 10c, 10d and a stepped section 10e between the first and second bores, and a porcelain insulator 20 having a largest-diameter section 20d, which is accommodated in the first bore and a small-diameter section 20e accommodated in the second bore. The first and second bores of the metal shell have a dimensional relationship, lying in a value equal to or less than 1.8 mm, which is expressed as (D1-D2)/2 where D1 represents an inner diameter of the first bore of the metal shell and D2 represents an inner diameter of the second bore of the metal shell.