An ignition plug (100, 100a) comprising: a tubular insulator having an axial hole extending in a direction of an axial line; a metallic shell (50, 50a) 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 (50, 50a) and facing the center electrode, wherein the metallic shell (50, 50a) has a threaded portion (57, 57d, 57u) to be engaged with a thread ridge (682, 682d, 682u) of a mounting hole (680, 680a) of an internal combustion engine (600, 600a), 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 (50, 50a) extending from a rear end (57r, 57ru) of the threaded portion (57, 57d, 57u) to a forward end (57f, 57fd) of the threaded portion (57, 57d, 57u), Sa is a surface area of that portion of the metallic shell (50, 50a) which is to be exposed to combustion gas of the internal combustion engine (600, 600a); 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 (50, 50a) 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 (100, 100a) 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 from a forward end 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 (50, 50a). a tubular insulator having an axial hole extending in a direction of an axial line; a metallic shell (50, 50a) 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 (50, 50a) and facing the center electrode, wherein the metallic shell (50, 50a) has a threaded portion (57, 57d, 57u) to be engaged with a thread ridge (682, 682d, 682u) of a mounting hole (680, 680a) of an internal combustion engine (600, 600a), 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 (50, 50a) extending from a rear end (57r, 57ru) of the threaded portion (57, 57d, 57u) to a forward end (57f, 57fd) of the threaded portion (57, 57d, 57u), Sa is a surface area of that portion of the metallic shell (50, 50a) which is to be exposed to combustion gas of the internal combustion engine (600, 600a); 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 (50, 50a) 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 (100, 100a) 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 from a forward end 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 (50, 50a).