A spark plug (101 to 107) comprising: a metal shell that has a substantially tubular shape and that is provided with a ground electrode at a front side of the metal shell ; an insulator that has a tubular shape and that is held in the metal shell , the insulator having an axial hole provided therein, the axial hole including a small-diameter portion and a large-diameter portion that has a diameter greater than a diameter of the small-diameter portion and that is connected to a back end of the small-diameter portion with a step portion provided therebetween; a resistor disposed in the large-diameter portion ; a center electrode including a flange portion that protrudes in a radial direction in the large-diameter portion and that is in contact with the step portion , a leg portion that extends toward the front side from the flange portion and that is disposed in the small-diameter portion , and a head portion that extends toward a back side from the flange portion ; and a conductive sealing member that is disposed in the large-diameter portion and that electrically connects the center electrode to the resistor , wherein the center electrode is formed by joining a conductive portion made of a conductive material and an insulating portion made of an insulating material, wherein the sealing member electrically connects the conductive portion to the resistor , characterized in that : the insulating portion has a rod-like shape and includes a protruding portion at a location on the back side of a back end of the sealing member , and the protruding portion is embedded in the resistor . a metal shell that has a substantially tubular shape and that is provided with a ground electrode at a front side of the metal shell ; an insulator that has a tubular shape and that is held in the metal shell , the insulator having an axial hole provided therein, the axial hole including a small-diameter portion and a large-diameter portion that has a diameter greater than a diameter of the small-diameter portion and that is connected to a back end of the small-diameter portion with a step portion provided therebetween; a resistor disposed in the large-diameter portion ; a center electrode including a flange portion that protrudes in a radial direction in the large-diameter portion and that is in contact with the step portion , a leg portion that extends toward the front side from the flange portion and that is disposed in the small-diameter portion , and a head portion that extends toward a back side from the flange portion ; and a flange portion that protrudes in a radial direction in the large-diameter portion and that is in contact with the step portion , a leg portion that extends toward the front side from the flange portion and that is disposed in the small-diameter portion , and a head portion that extends toward a back side from the flange portion ; and a conductive sealing member that is disposed in the large-diameter portion and that electrically connects the center electrode to the resistor , wherein the center electrode is formed by joining a conductive portion made of a conductive material and an insulating portion made of an insulating material, wherein the sealing member electrically connects the conductive portion to the resistor , characterized in that : the insulating portion has a rod-like shape and includes a protruding portion at a location on the back side of a back end of the sealing member , and the protruding portion is embedded in the resistor . <EOS>