An ignition plug comprising: a tubular insulator having an axial hole extending in a direction of an axial line (CL); a metallic shell disposed around an outer circumference of the insulator ; a center electrode disposed in the axial hole of the insulator ; and a ground electrode connected to a forward end of the metallic shell and facing the center electrode , wherein the metallic shell has a threaded portion to be engaged with a thread ridge of a mounting hole of an internal combustion engine , and a relational expression Ss/(Sa + Sb) ≥ 2.6 is satisfied, where Ss is a surface area of an outer circumferential surface of the metallic shell extending from a rear end of the threaded portion to a forward end of the threaded portion , Sa is a surface area of that portion of the metallic shell which is to be exposed to combustion gas of the internal combustion engine ; and Sb is a surface area of that portion of the insulator which is to be exposed to the combustion gas; wherein the metallic shell has an inside-diameter-reducing portion whose inside diameter reduces toward a forward-end side; the insulator has an outside-diameter-reducing portion whose outside diameter reduces toward the forward-end side; the ignition plug has a packing in contact with the outside-diameter-reducing portion and with the inside-diameter-reducing portion , or the outside-diameter-reducing portion is in direct contact with the inside-diameter-reducing portion ; and a relational expression F ≥ 5.0 mm is satisfied, where F is a distance in the direction of the axial line (CL) from a forward end (P3) of a contact portion between the outer circumferential surface of the insulator and the inside-diameter-reducing portion or the packing to the forward end of the metallic shell . a tubular insulator having an axial hole extending in a direction of an axial line (CL); a metallic shell disposed around an outer circumference of the insulator ; a center electrode disposed in the axial hole of the insulator ; and a ground electrode connected to a forward end of the metallic shell and facing the center electrode , wherein the metallic shell has a threaded portion to be engaged with a thread ridge of a mounting hole of an internal combustion engine , and a relational expression Ss/(Sa + Sb) ≥ 2.6 is satisfied, where Ss is a surface area of an outer circumferential surface of the metallic shell extending from a rear end of the threaded portion to a forward end of the threaded portion , Sa is a surface area of that portion of the metallic shell which is to be exposed to combustion gas of the internal combustion engine ; and Sb is a surface area of that portion of the insulator which is to be exposed to the combustion gas; wherein the metallic shell has an inside-diameter-reducing portion whose inside diameter reduces toward a forward-end side; the insulator has an outside-diameter-reducing portion whose outside diameter reduces toward the forward-end side; the ignition plug has a packing in contact with the outside-diameter-reducing portion and with the inside-diameter-reducing portion , or the outside-diameter-reducing portion is in direct contact with the inside-diameter-reducing portion ; and a relational expression F ≥ 5.0 mm is satisfied, where F is a distance in the direction of the axial line (CL) from a forward end (P3) of a contact portion between the outer circumferential surface of the insulator and the inside-diameter-reducing portion or the packing to the forward end of the metallic shell . <EOS>