A glow plug, comprising: a heater including a tubular sheath tube (810, 810b, 810c) that extends in an axial direction and that has a closed front end, a heating coil (820, 820a) accommodated in the sheath tube (810, 810b, 810c) with a front-end portion (822, 822a) thereof joined to a front-end portion of the sheath tube (810, 810b, 810c), and an insulator filled in the sheath tube (810, 810b, 810c) around the heating coil (820, 820a), wherein the heating coil (820, 820a) contains at least one selected from tungsten and molybdenum (Mo) as a main component and contains an additional element that is at least one element selected from potassium, aluminum (Al), silicon (Si), lanthanum (La), thorium (Th), and cerium (Ce), wherein the sheath tube (810, 810b, 810c) contains at least one metal selected from nickel (Ni) and iron (Fe), wherein a melt portion that is in contact with an outer surface of the front-end portion (822, 822a) of the heating coil (820, 820a) and that contains at least the same material as that of a portion of the sheath tube (810, 810b, 810c) other than the melt portion is formed at the front-end portion of the sheath tube (810, 810b, 810c), and wherein, when a cross section of a wire forming the heating coil (820, 820a) is viewed at the front-end portion (822, 822a) of the heating coil (820, 820a), the additional element exists at a crystal grain boundary of the main component at least in a surface layer extending from the outer surface of the heating coil (820, 820a) to a length of a quarter of a diameter of the wire, characterized in that a content of the additional element of the heating coil is between 5 ppm or more and 200 ppm or less. a heater including a tubular sheath tube (810, 810b, 810c) that extends in an axial direction and that has a closed front end, a heating coil (820, 820a) accommodated in the sheath tube (810, 810b, 810c) with a front-end portion (822, 822a) thereof joined to a front-end portion of the sheath tube (810, 810b, 810c), and an insulator filled in the sheath tube (810, 810b, 810c) around the heating coil (820, 820a), wherein the heating coil (820, 820a) contains at least one selected from tungsten and molybdenum (Mo) as a main component and contains an additional element that is at least one element selected from potassium, aluminum (Al), silicon (Si), lanthanum (La), thorium (Th), and cerium (Ce), wherein the sheath tube (810, 810b, 810c) contains at least one metal selected from nickel (Ni) and iron (Fe), wherein a melt portion that is in contact with an outer surface of the front-end portion (822, 822a) of the heating coil (820, 820a) and that contains at least the same material as that of a portion of the sheath tube (810, 810b, 810c) other than the melt portion is formed at the front-end portion of the sheath tube (810, 810b, 810c), and wherein, when a cross section of a wire forming the heating coil (820, 820a) is viewed at the front-end portion (822, 822a) of the heating coil (820, 820a), the additional element exists at a crystal grain boundary of the main component at least in a surface layer extending from the outer surface of the heating coil (820, 820a) to a length of a quarter of a diameter of the wire, characterized in that a content of the additional element of the heating coil is between 5 ppm or more and 200 ppm or less.