Spark plug

1. 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 S n 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 a n , a center of the axial hole is defined as a point O n , a line which passes through the points a n and O n is defined as a line L n , a diameter of the axial hole is represented by 2R n , a diameter of the accommodated portion is represented by 2r n , a distance between the point a n and the point O n is represented by H aon , and the point a n which satisfies a relation H aon ≧0.8(R n −r n ) is defined as a point A n ,

on the sectional image S 1 ,

a line which is obtained by rotating a line L 1 by 45° about the point O 1 is defined as a line L 1+ , a line which is obtained by rotating the line L 1 by −95° about the point O 1 is defined as a line L 1− , four areas which are surrounded by a plane X + containing the line L 1+ and the axis, a plane X − containing the line L 1− and the axis, and the inner circumferential surface of the insulator are areas T 1 , T 2 , T 3 , and T 4 , respectively, and

when successive points A m to A (m+k) , m and k being natural numbers, are present in a specific area selected from the areas T 1 , T 2 , T 3 , and T 4 , a group containing these points A m to A (m+k) is defined as a group B m,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 B m,y are present in two areas which are selected from the areas T 1 to T 4 and which are located at symmetrical positions.

2. A spark plug according to claim 1 , wherein an inter-bend distance D, which is a distance along the direction of the axis between a sectional image containing a point A s , which is the first point A n counted from the rear end of the accommodated portion, and a sectional image containing a point A e , which is the last point A n counted from the rear end of the accommodated portion, is 5 mm or greater.

3. A spark plug according to claim 1 wherein the groups B m,y are present in at least three areas selected from the areas T 1 to T 4 .

4. A spark plug according to claim 1 , wherein the maximum value of y is at least 4.

5. A spark plug according to claim 1 , wherein the groups B m,y are present in all the areas T 1 to T 4 .

6. A spark plug according to claim 1 , wherein the maximum value of y is at least 5.

7. A spark plug according to claim 1 , wherein an intermediate-diameter portion diameter, which is a diameter of the axial hole at a position where the forward end portion is disposed, is 2.9 mm or less.

8. A spark plug according to claim 1 , wherein the inter-bend distance D is 7 mm or greater.

9. A spark plug according to claim 1 , wherein the inter-bend distance D is 10 mm or greater.