A spark plug comprising: an insulator that has an axial hole extending in a direction of an axial line; a center electrode disposed at a front end side of the axial hole; a metal terminal disposed at a rear end side of the axial hole; a connecting portion which is electrically connected to the center electrode and the metal terminal within the axial hole; a metal shell accommodating the insulator therein; and a ground electrode, a first end portion of which is joined to a front end portion of the metal shell, and a second end portion of which is disposed apart from the center electrode so as to define a gap therebetween, wherein at least either of the center electrode and the ground electrode has a tip which defines the gap, wherein the tip contains Ir, Rh and Ru in a total amount of 95 mass % or more with respect to the whole mass amount thereof, and the contents of Rh and Ru in mass % lie within an area that is surrounded by a line connecting point A having the coordinates 6, 1, point B having the coordinates 6, 15, point C having the coordinates 33, 18, point D having the coordinates 33, 4 and the point A having the coordinates 6, 1 in this order, or lie on the line, wherein the coordinates of the points are defined in a coordinate system with Rh in mass% on the abscissa and Ru in mass% on the ordinate, characterized in that the tip satisfies a relation of 1.5 ≤ Has/Han ≤ 2.2, wherein Has is a Vickers hardness at a cut surface of the tip which results when the tip is cut along a plane which includes the axial line, wherein Has is the arithmetic mean value of the Vickers hardness measured at several points at the cut surface in accordance with JIS Z 2244, and Han is a Vickers hardness at the cut surface after the tip is placed in a furnace of an Ar atmosphere with Ar caused to flow at a rate of 2 liter/min and heated to and held at 1300°C for 10 hours and then cooled down by stopping the heating of the tip and keeping Ar flowing into the furnace at the same rate even after the heating of the tip has been stopped, wherein Han is the arithmetic mean value of the Vickers hardness measured at several points at the cut surface in accordance with JIS Z 2244, wherein the tip is subjected to a heat treatment, in which the tip is held at predetermined temperature lying within the range of heat treatment temperatures of 800 to 1500°C, at which recrystallization does not occur and strain is removed to some extent, for predetermined lengths of time lying within the range of holding time of 0 second to 1 hour so as to control their hardness ratios (Has/Han) to satisfy the relation of 1.5 ≤ Has/Han ≤ 2.2. an insulator that has an axial hole extending in a direction of an axial line; a center electrode disposed at a front end side of the axial hole; a metal terminal disposed at a rear end side of the axial hole; a connecting portion which is electrically connected to the center electrode and the metal terminal within the axial hole; a metal shell accommodating the insulator therein; and a ground electrode, a first end portion of which is joined to a front end portion of the metal shell, and a second end portion of which is disposed apart from the center electrode so as to define a gap therebetween, wherein at least either of the center electrode and the ground electrode has a tip which defines the gap, wherein the tip contains Ir, Rh and Ru in a total amount of 95 mass % or more with respect to the whole mass amount thereof, and the contents of Rh and Ru in mass % lie within an area that is surrounded by a line connecting point A having the coordinates 6, 1, point B having the coordinates 6, 15, point C having the coordinates 33, 18, point D having the coordinates 33, 4 and the point A having the coordinates 6, 1 in this order, or lie on the line, wherein the coordinates of the points are defined in a coordinate system with Rh in mass% on the abscissa and Ru in mass% on the ordinate, characterized in that the tip satisfies a relation of 1.5 ≤ Has/Han ≤ 2.2, wherein Has is a Vickers hardness at a cut surface of the tip which results when the tip is cut along a plane which includes the axial line, wherein Has is the arithmetic mean value of the Vickers hardness measured at several points at the cut surface in accordance with JIS Z 2244, and Han is a Vickers hardness at the cut surface after the tip is placed in a furnace of an Ar atmosphere with Ar caused to flow at a rate of 2 liter/min and heated to and held at 1300°C for 10 hours and then cooled down by stopping the heating of the tip and keeping Ar flowing into the furnace at the same rate even after the heating of the tip has been stopped, wherein Han is the arithmetic mean value of the Vickers hardness measured at several points at the cut surface in accordance with JIS Z 2244, wherein the tip is subjected to a heat treatment, in which the tip is held at predetermined temperature lying within the range of heat treatment temperatures of 800 to 1500°C, at which recrystallization does not occur and strain is removed to some extent, for predetermined lengths of time lying within the range of holding time of 0 second to 1 hour so as to control their hardness ratios (Has/Han) to satisfy the relation of 1.5 ≤ Has/Han ≤ 2.2.