Spark plug

1. A spark plug comprising:

an insulator extending along an axial line from a front side to a rear side and having a step portion of which an outer diameter reduces toward the front side in the axial-line direction; and

a cylindrical metal shell having, on an inner circumference thereof, a ledge portion of which an inner diameter reduces toward the front side in the axial-line direction, the metal shell holding the insulator from an outer circumferential side in a state in which the step portion is engaged with the ledge portion via a packing, wherein

the packing includes a base material, and a metal layer formed on a surface of the base material and contacting with the insulator and the metal shell,

the base material includes a first surface where a part of the metal layer that contacts with the insulator is formed, a second surface on a side opposite to the first surface, and third surfaces connecting the first surface and the second surface, and

in a cross section including the axial line,

of distances between end points of the metal layer on a first contact surface of the insulator that contacts with the metal layer and first points which are intersections of the first contact surface and first perpendiculars extending to the first contact surface from first corners at which the first surface and the third surfaces intersect each other, a shorter distance positioned on the front side in the axial-line direction is defined as distance A 1 , and a shorter distance positioned on the rear side in the axial-line direction is defined as distance A 2 , and

at least one of the distance A 1 and the distance A 2 is longer than a thickness of the metal layer in a direction perpendicular to the first perpendicular at a middle position on the third surface that corresponds to half a length measured along the third surface from the first corner to a second corner at which the second surface and the third surface intersect each other, on a side where the corresponding one of the distance A 1 and the distance A 2 is measured.

2. The spark plug according to claim 1 , wherein

in the cross section including the axial line, at least one of the distance A 1 and the distance A 2 is longer than a distance C between the first surface and the first contact surface.

3. The spark plug according to claim 1 , wherein

of the metal layer, a part contacting with the metal shell is formed at the second surface, and

in the cross section including the axial line,

of distances between end points of the metal layer on a second contact surface of the metal shell that contacts with the metal layer and second points which are intersections of the second contact surface and second perpendiculars extending from the second corner to the second contact surface, a shorter distance positioned on the front side in the axial-line direction is defined as distance B 1 , and a shorter distance positioned on the rear side in the axial-line direction is defined as distance B 2 , and

at least one of the distance B 1 and the distance B 2 is longer than a thickness of the metal layer in a direction perpendicular to the second perpendicular at the middle position on a side where the corresponding one of the distance B 1 and the distance B 2 is measured.

4. The spark plug according to claim 3 , wherein

in the cross section including the axial line, at least one of the distance B 1 and the distance B 2 is longer than a distance D between the second surface and the second contact surface.