A spark plug comprising: a shaft-shaped center electrode; a tubular insulator which accommodates at least a rear end-side portion of the center electrode in an inner portion of the insulator; a grounding electrode which is disposed while having a gap between a tip portion of the center electrode and the grounding electrode; and a tubular metal shell including a through-hole, particularly a cylindrical hole, in which the insulator is accommodated, wherein the grounding electrode is fixed to an inner wall surface of the through-hole of the metal shell, wherein the grounding electrode is fixed to the metal shell via a melting portion in which the grounding electrode and the metal shell are melted to each other, characterized in that in a cross section including a bottom portion of the melting portion, which is the rearmost end-side portion in the melting portion, and a central axis of the through-hole, in or of the melting portion, a melting depth which is a distance in a central axis direction of the through-hole between the bottom portion of the melting portion and a virtual straight line including an outline of a tip-side surface of the grounding electrode is 40% or more of a thickness of the grounding electrode in the central axis direction, and an area (Sm) of the shell-side portion which is positioned at an outer circumferential side of the metal shell from a virtual straight line connecting endpoints (EPa, EPb) of the inner wall surface of the metal shell which are positioned at a tip side and a rear end side of the melting portion in the central axis direction is 20% or more of the entire area of the melting portion. a shaft-shaped center electrode; a tubular insulator which accommodates at least a rear end-side portion of the center electrode in an inner portion of the insulator; a grounding electrode which is disposed while having a gap between a tip portion of the center electrode and the grounding electrode; and a tubular metal shell including a through-hole, particularly a cylindrical hole, in which the insulator is accommodated, wherein the grounding electrode is fixed to an inner wall surface of the through-hole of the metal shell, wherein the grounding electrode is fixed to the metal shell via a melting portion in which the grounding electrode and the metal shell are melted to each other, characterized in that in a cross section including a bottom portion of the melting portion, which is the rearmost end-side portion in the melting portion, and a central axis of the through-hole, in or of the melting portion, a melting depth which is a distance in a central axis direction of the through-hole between the bottom portion of the melting portion and a virtual straight line including an outline of a tip-side surface of the grounding electrode is 40% or more of a thickness of the grounding electrode in the central axis direction, and an area (Sm) of the shell-side portion which is positioned at an outer circumferential side of the metal shell from a virtual straight line connecting endpoints (EPa, EPb) of the inner wall surface of the metal shell which are positioned at a tip side and a rear end side of the melting portion in the central axis direction is 20% or more of the entire area of the melting portion.