Spark plug

A spark plug comprising:

• an insulator having an axial hole extending in a direction of an axis;
• a center electrode held at one end of the axial hole;
• a metallic terminal which has an accommodated portion accommodated in the axial hole and which is held at the other end of the insulator;
• and
• a connecting portion which electrically connects the center electrode and the metallic terminal within the axial hole, wherein:
• the side of the axial hole where the metallic terminal is held is defined as a rear end side with respect to the direction of the axis;
• a connection member which constitutes the connecting portion is provided between an outer circumferential surface of a forward end portion of the accommodated portion and an inner circumferential surface of the insulator;
• and
• in sectional images orthogonal to the axis which are captured at intervals of 0.5 mm from a rear end of the accommodated portion toward a forward end thereof up to a rear end position where the connection member is present,
• on a sectional image Sn at the n-th section, n being a natural number, counted from the rear end of the accommodated portion,
• a center of the accommodated portion is defined as a point an, a center of the axial hole is defined as a point On, a line which passes through the points an and On is defined as a line Ln, a diameter of the axial hole is represented by 2Rn, a diameter of the accommodated portion is represented by 2rn, a distance between the point an and the point On is represented by Haon, and the point an which satisfies a relation Haon≧0.8(Rn−rn) is defined as a point An,
• on the sectional image S1,
• a line which is obtained by rotating a line L1 by 45° about the point O1 is defined as a line L1+, a line which is obtained by rotating the line L1 by −95° about the point O1 is defined as a line L1−, four areas which are surrounded by a plane X+ containing the line L1+ and the axis, a plane X− containing the line L1− and the axis, and the inner circumferential surface of the insulator are areas T1, T2, T3, and T4, respectively, and
• when successive points Am to A(m+k), m and k being natural numbers, are present in a specific area selected from the areas T1, T2, T3, and T4, a group containing these points Am to A(m+k) is defined as a group Bm,y, where y is a natural number and means that the group is the y-th group counted from the rear end side of the accommodated portion,
• wherein the maximum value of y is at least 3, and at least two of the groups Bm,y are present in two areas which are selected from the areas T1 to T4 and which are located at symmetrical positions.