A spark plug for an internal combustion engine, comprising: a rod-like center electrode extending in a direction of an axis; a substantially cylindrical insulator provided externally of an outer circumference of the center electrode; a substantially cylindrical metallic shell provided externally of an outer circumference of the insulator; and a ground electrode extending from a front end portion of the metallic shell and defining, in cooperation with the center electrode, a gap between a distal end portion thereof and a front end portion of the center electrode; the metallic shell having, on an outer circumferential surface thereof: a threaded portion adapted to be threadingly engaged with a mounting hole of a head of an internal combustion engine, a screw neck located rearward of the threaded portion, a diameter-expanded portion located rearward of the screw neck and greater in diameter than the screw neck, and a seat portion located between the screw neck and the diameter-expanded portion, wherein at the time of the threaded portion being threadingly engaged with the mounting hole of the head of the internal combustion engine, the seat portion comes in close contact with the head, wherein: the threaded portion has a thread diameter of M14, the seat portion has a Vickers hardness of 250 Hv or less and is higher in hardness than a portion of the head which comes into contact with the seat portion, the metallic shell has, on an outer circumferential surface thereof, a connection portion which connects a front end of the seat portion and a rear end of the screw neck and forms, with the axis, an angle greater than an angle between the seat portion and the axis as viewed on a section which contains the axis, and when A represents an outside diameter of the diameter-expanded portion, B represents a smallest outside diameter of the screw neck, and C represents an outside diameter of a boundary between the seat portion and the connection portion, the following expressions and are satisfied: C − B / 2 ≥ 0.3 mm and A − C / 2 ≥ 0.7 mm a rod-like center electrode extending in a direction of an axis; a substantially cylindrical insulator provided externally of an outer circumference of the center electrode; a substantially cylindrical metallic shell provided externally of an outer circumference of the insulator; and a ground electrode extending from a front end portion of the metallic shell and defining, in cooperation with the center electrode, a gap between a distal end portion thereof and a front end portion of the center electrode; a threaded portion adapted to be threadingly engaged with a mounting hole of a head of an internal combustion engine, a screw neck located rearward of the threaded portion, a diameter-expanded portion located rearward of the screw neck and greater in diameter than the screw neck, and a seat portion located between the screw neck and the diameter-expanded portion, the threaded portion has a thread diameter of M14, the seat portion has a Vickers hardness of 250 Hv or less and is higher in hardness than a portion of the head which comes into contact with the seat portion, the metallic shell has, on an outer circumferential surface thereof, a connection portion which connects a front end of the seat portion and a rear end of the screw neck and forms, with the axis, an angle greater than an angle between the seat portion and the axis as viewed on a section which contains the axis, and when A represents an outside diameter of the diameter-expanded portion, B represents a smallest outside diameter of the screw neck, and C represents an outside diameter of a boundary between the seat portion and the connection portion, the following expressions and are satisfied: C − B / 2 ≥ 0.3 mm and A − C / 2 ≥ 0.7 mm