A spark plug is provided which is designed to be compact without sacrificing a mechanical strength of a porcelain insulator. The spark plug includes a metal shell having a base end and a top end. The porcelain insulator is made of a hollow cylinder which includes a body and an insulator head. The body is retained within the metal shell. The insulator head extends from the base end of the metal shell in a lengthwise direction of the porcelain insulator and has a length made up of a major body leading to the body of the porcelain insulator and an end portion lying far away from the body. The major body has an outer diameter D1, an inner diameter D2, and a section modulus Z at a smallest-outer diameter portion thereof which meet relations of 7.1 mm≦D1≦8.8 mm, D2≧2.8 mm, and Z≧33 mm3.