A glow plug comprising: a metallic shell having a through hole extending in the direction of an axial line; a sleeve inserted into the through hole of the metallic shell and fixed to the metallic shell; a ceramic heater inserted into the sleeve and fixed to the sleeve; a center rod (30, 30b, 30c, 30d, 30e, 30f) formed of a metal, inserted into the through hole of the metallic shell, and electrically connected to the ceramic heater; and a packing sandwiched between an inner circumferential surface of the metallic shell and an outer circumferential surface of the center rod (30, 30b, 30c, 30d, 30e, 30f), wherein the center rod (30, 30b, 30c, 30d, 30e, 30f) includes one or more neck portions (410, 420) which extend in the direction of the axial line and in which a plurality of grooves each extending on the outer circumferential surface of the center rod (30, 30b, 30c, 30d, 30e, 30f) in the circumferential direction thereof are successively arranged in the direction of the axial line; characterized in that: the following relational expression is satisfied: Lb > 19.7 mm 3 Da 3 × La where La is a distance between a first position and a second position in the direction of the axial line, the first position being a position of a center, in the direction of the axial line, of a contact area (Ac) between the packing and the outer circumferential surface of the center rod (30, 30b, 30c, 30d, 30e, 30f), the second position being a position of a rearmost end of a contact area (Ae) between the ceramic heater and the sleeve, Lb is a distance in the direction of the axial line between the first position and a forward end of a forwardmost neck portion of the one or more neck portions (410, 420), and Da is an outer diameter of the ceramic heater at the second position. a metallic shell having a through hole extending in the direction of an axial line; a sleeve inserted into the through hole of the metallic shell and fixed to the metallic shell; a ceramic heater inserted into the sleeve and fixed to the sleeve; a center rod (30, 30b, 30c, 30d, 30e, 30f) formed of a metal, inserted into the through hole of the metallic shell, and electrically connected to the ceramic heater; and a packing sandwiched between an inner circumferential surface of the metallic shell and an outer circumferential surface of the center rod (30, 30b, 30c, 30d, 30e, 30f), wherein the center rod (30, 30b, 30c, 30d, 30e, 30f) includes one or more neck portions (410, 420) which extend in the direction of the axial line and in which a plurality of grooves each extending on the outer circumferential surface of the center rod (30, 30b, 30c, 30d, 30e, 30f) in the circumferential direction thereof are successively arranged in the direction of the axial line; characterized in that: the following relational expression is satisfied: Lb > 19.7 mm 3 Da 3 × La where La is a distance between a first position and a second position in the direction of the axial line, the first position being a position of a center, in the direction of the axial line, of a contact area (Ac) between the packing and the outer circumferential surface of the center rod (30, 30b, 30c, 30d, 30e, 30f), the second position being a position of a rearmost end of a contact area (Ae) between the ceramic heater and the sleeve, Lb is a distance in the direction of the axial line between the first position and a forward end of a forwardmost neck portion of the one or more neck portions (410, 420), and Da is an outer diameter of the ceramic heater at the second position.