A spark plug has a housing of a cylindrical shape and an insulator of a cylindrical shape supported inside the housing. A position of an insulator distal end at a distal end of the insulator is arranged equal to or forward of a position of a distal end of the housing in an axial direction of the spark plug. The spark plug satisfies a relationship designated by an equation, −0.064L2+0.26E+0.53≤T≤0.07L2−0.35L+1.36, where T indicates a thickness of the insulator distal end of the insulator, and L indicates an insulator projection length measured from the position of the distal end of the housing to the position at the insulator distal end surface of the insulator.