Ignition plug

1. An ignition plug comprising:

an insulator that includes a through hole;

a center electrode that includes a first discharge surface and that is held at a front end side of the through hole;

a metal shell that is disposed around the insulator in a radial direction and that holds the insulator;

a bar-shaped ground electrode body that includes a joining end surface and a free end surface, the joining end surface being joined to a front end of the metal shell, the free end surface being positioned opposite to the joining end surface;

a ground electrode tip that is disposed along a side surface of the ground electrode body opposing the first discharge surface near the free end surface of the ground electrode body, and that includes a second discharge surface opposing the first discharge surface; and

a welding portion that is disposed between the ground electrode tip and the ground electrode body, and that includes a component of the ground electrode tip and a component of the ground electrode body,

wherein, in a section which extends through a center of gravity of the second discharge surface, which is perpendicular to the second discharge surface, and which is parallel to an axial line of the ground electrode body,

a direction from the center of gravity of the second discharge surface to the free end surface along the second discharge surface is a first direction, and a direction opposite to the first direction is a second direction,

of an end, located in the first direction, of a boundary between the welding portion and the ground electrode tip and an end, located in the first direction, of a boundary between the welding portion and the ground electrode body, the end that is positioned towards a side in the second direction is a first end, and

an end of the ground electrode tip located in the second direction is a second end;

wherein an end of the welding portion located in the first direction is exposed at the free end surface,

the welding portion extends along the axial line of the ground electrode body, and

wherein, over an entire sub-range of a range, the range extending from the first end to the second end, the sub-range being ¼ of the range nearest the second end, a length L 1 of the ground electrode tip in a direction perpendicular to the first direction and a length L 2 of the welding portion in the direction perpendicular to the first direction satisfy (L 2 /L 1 )≧0.25.

2. The ignition plug according to claim 1 , wherein, in the section, further, over an entirety of the range, the length L 1 of the ground electrode tip in the direction perpendicular to the first direction and the length L 2 of the welding portion in the direction perpendicular to the first direction satisfy (L 2 /L 1 )≧0.25.

3. The ignition plug according to claim 1 , wherein, in the section, further, a length L 3 from the second end to an end of the welding portion located in the second direction is greater than or equal to 0.1 mm.

4. The ignition plug according to claim 1 , wherein, in the section, further, in the entire sub-range, the length L 1 of the ground electrode tip in the direction perpendicular to the first direction and the length L 2 of the welding portion in the direction perpendicular to the first direction satisfy (L 2 /L 1 )≦0.5.

5. The ignition plug according to claim 1 , wherein an end of the ground electrode tip located in the first direction is positioned more in the second direction than the free end surface of the ground electrode body.