An ignition plug capable of improving ignition performance while preventing pre-ignition includes a center electrode, a bottomed cylindrical insulator enclosing a front end portion of the center electrode and a cylindrical metal shell holding therein the insulator. The insulator has, on a front end side thereof, a protruding portion protruding from and located frontward of a front end of the metal shell. In the ignition plug, a difference between an outer diameter of the center electrode and an inner diameter of the insulator is 0.01 mm or more; an axial length of the protruding portion is 6.7 mm to 12.7 mm; and the following condition is satisfied: 1.07≤S/V1≤1.35 where S is an outer surface area of the protruding portion; and V1 is a volume of the protruding portion.