Aglow plug comprising: a metallic shell having a through hole extending in the direction of an axial line, wherein the axial line corresponds to the center axis of the glow plug; a ceramic heater inserted into the through hole of the metallic shell and held at a forward end of the metallic shell; and a center rod (30, 30c, 30d, 30e, 30f) formed of a metal, inserted into the through hole of the metallic shell, and electrically connected to the ceramic heater, wherein the center rod (30, 30c, 30d, 30e, 30f) includes one or more neck portions which extend in the direction of the axial line and in which a plurality of grooves each extending on an outer circumferential surface of the center rod (30, 30c, 30d, 30e, 30f) in the circumferential direction thereof are successively arranged in the direction of the axial line; of the one or more neck portions, a forwardmost neck portion has a portion disposed forward of a middle position in the direction of the axial line between a forward end and a rear end of the center rod (30, 30c, 30d, 30e, 30f), wherein a forward direction is considered as being the direction toward the ceramic heater and the middle position is the middle position, in the direction parallel to the axial line between the forward end and the rear end of the center rod characterized in that in the forwardmost neck portion, the plurality of grooves are arranged in the direction of the axial line at a pitch (Pt) of less than 1.1 mm; on a flat cross section containing the axial line, each groove has a forward sloping surface which slopes forward to be oblique to the direction of the axial line and a rear sloping surface which slopes rearward to be oblique to the direction of the axial line; and in the forwardmost neck portion, the forward sloping surface and the rear sloping surface form an angle (Ang) of less than 60°. a metallic shell having a through hole extending in the direction of an axial line, wherein the axial line corresponds to the center axis of the glow plug; a ceramic heater inserted into the through hole of the metallic shell and held at a forward end of the metallic shell; and a center rod (30, 30c, 30d, 30e, 30f) formed of a metal, inserted into the through hole of the metallic shell, and electrically connected to the ceramic heater, wherein the center rod (30, 30c, 30d, 30e, 30f) includes one or more neck portions which extend in the direction of the axial line and in which a plurality of grooves each extending on an outer circumferential surface of the center rod (30, 30c, 30d, 30e, 30f) in the circumferential direction thereof are successively arranged in the direction of the axial line; of the one or more neck portions, a forwardmost neck portion has a portion disposed forward of a middle position in the direction of the axial line between a forward end and a rear end of the center rod (30, 30c, 30d, 30e, 30f), wherein a forward direction is considered as being the direction toward the ceramic heater and the middle position is the middle position, in the direction parallel to the axial line between the forward end and the rear end of the center rod in the forwardmost neck portion, the plurality of grooves are arranged in the direction of the axial line at a pitch (Pt) of less than 1.1 mm; on a flat cross section containing the axial line, each groove has a forward sloping surface which slopes forward to be oblique to the direction of the axial line and a rear sloping surface which slopes rearward to be oblique to the direction of the axial line; and in the forwardmost neck portion, the forward sloping surface and the rear sloping surface form an angle (Ang) of less than 60°.