An ignition device is provided, which can boost an electromagnetic wave supplied by a resonance structure, and cause a discharge by enhancing a potential difference between a discharge electrode and a ground electrode, and even though such a structure of the ignition device, a downsize and a thickness reduction, specifically, the thickness reduction can be achieved. On a main surface of a rectangular insulting substrate (2), an input electrode (3), a coupling electrode (4), a discharge electrode (6), and a ground electrode (7), are provided. The input electrode (3) is connected to an outside terminal on one shorter side. The coupling electrode (4) is capacity-coupled with the input electrode (3). The discharge electrode (6) is connected to the coupling electrode (4) on the other shorter side through a coupling line (5). The ground electrode (7) is, on both longer sides of the main surface of the rectangular insulating substrate (2), capacity-coupled with the coupling electrode (4) and capacity-coupled with the coupling line (5), and extended to the other shorter side. A resonance circuit includes a capacitor constituted by the capacity coupling and an inductor constituted by the coupling line (5). Thereby, the electromagnetic wave supplied from the outside terminal into the input electrode is resonated, a potential difference between the discharge electrode (6) and the ground electrode (7) is enhanced, and then, a discharge is caused.

The spark plug includes: an insulator having a through hole that penetrates therethrough along an axial direction; a center electrode disposed on one end side of the through hole; a metal terminal disposed on the other end side of the through hole; and a conductive seal layer connected to at least one of the center electrode and the metal terminal. The conductive seal layer contains glass and a Cu—Zn alloy. A volumetric percentage of the Cu—Zn alloy in the conductive seal layer is greater than or equal to 44% and not greater than 55%.

The spark plug includes: an insulator having a through hole that penetrates therethrough along an axial direction; a center electrode disposed on one end side of the through hole; a metal terminal disposed on the other end side of the through hole; and a conductive seal layer connected to at least one of the center electrode and the metal terminal. The conductive seal layer contains glass and a Cu—Zn alloy. A volumetric percentage of the Cu—Zn alloy in the conductive seal layer is greater than or equal to 44% and not greater than 55%.

A spark plug includes a housing, an insulator inside the housing, a center electrode inside the insulator, and a connecting pin inside the insulator. At least one fillet is formed at a change of cross section at the housing, at the insulator, at the center electrode, and/or at the connecting pin. The fillet, viewed in cross section, includes a first leg and a second leg at an angle to the first leg. The length of the first leg is greater than the length of the second leg.

A spark plug includes a housing, an insulator inside the housing, a center electrode inside the insulator, and a connecting pin inside the insulator. At least one fillet is formed at a change of cross section at the housing, at the insulator, at the center electrode, and/or at the connecting pin. The fillet, viewed in cross section, includes a first leg and a second leg at an angle to the first leg. The length of the first leg is greater than the length of the second leg.

An ignition plug includes an insulator that includes a through hole; a center electrode; a metal shell that holds the insulator; a bar-shaped ground electrode body; an electrode tip that is disposed along a side surface of the ground electrode body opposing a discharge surface of the center electrode; and a welding portion that is disposed between the ground electrode tip and the ground electrode body. Over 1/4 of a range from a first end to a second end of the ground electrode tip, a length L1 of the ground electrode tip and a length L2 of the welding portion in the direction satisfy (L2/L1) ≧0.25.

An insulator is rendered less breakable. A spark plug insulator is a tube-shaped spark plug insulator having a through hole extending in a direction of an axial line. The spark plug insulator contains alumina as a main component and mullite at at least part of the insulator. Mullite is contained in only an inner circumferential surface of the tube-shaped spark plug insulator and in at least part of the inner circumferential surface of the spark plug insulator in an area extending toward a distal end from a portion having a largest outer diameter.

A spark plug includes a terminal electrode extending in an axial direction of the spark plug. The terminal electrode has an end in the axial direction, and has a recess formed in the end of the terminal electrode. The recess has a bottom surface and an identifier on the bottom surface. The recess has a side surface parallel to the axial direction.

A spark plug includes a terminal electrode extending in an axial direction of the spark plug. The terminal electrode has an end in the axial direction, and has a recess formed in the end of the terminal electrode. The recess has a bottom surface and an identifier on the bottom surface. The recess has a side surface parallel to the axial direction.

A spark plug includes a terminal electrode extending in an axial direction of the spark plug. The terminal electrode has an end in the axial direction, and has a recess formed in the end of the terminal electrode. The recess has a bottom surface and an identifier on the bottom surface. The recess has a side surface parallel to the axial direction.