A glow plug comprising: a metallic shell having a through hole extending in the direction of an axial line (CL); 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 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 , wherein the center rod includes one or more neck portions which extend in the direction of the axial line (CL) and in which a plurality of grooves each extending on the outer circumferential surface of the center rod in the circumferential direction thereof are successively arranged in the direction of the axial line (CL); 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 (P1) and a second position (P2) in the direction of the axial line (CL), the first position (P1) being a position of a center, in the direction of the axial line (CL), of a contact area (Ac) between the packing and the outer circumferential surface of the center rod , the second position (P2) 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 (CL) between the first position (P1) and a forward end of a forwardmost neck portion of the one or more neck portions , and Da is an outer diameter of the ceramic heater at the second position (P2). a metallic shell having a through hole extending in the direction of an axial line (CL); 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 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 , wherein the center rod includes one or more neck portions which extend in the direction of the axial line (CL) and in which a plurality of grooves each extending on the outer circumferential surface of the center rod in the circumferential direction thereof are successively arranged in the direction of the axial line (CL); 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 (P1) and a second position (P2) in the direction of the axial line (CL), the first position (P1) being a position of a center, in the direction of the axial line (CL), of a contact area (Ac) between the packing and the outer circumferential surface of the center rod , the second position (P2) 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 (CL) between the first position (P1) and a forward end of a forwardmost neck portion of the one or more neck portions , and Da is an outer diameter of the ceramic heater at the second position (P2). <EOS>