Spark plug has center electrode having leg portion extending in an axis direction, brim portion located at rear end side with respect to the leg portion and protruding outwards in a radial direction with respect to the leg portion and connecting portion connecting the leg portion and the brim portion; insulator having penetration hole and supporting the center electrode; and seal member filling the penetration hole and fixing the brim portion and the insulator. The center electrode satisfies “(D1-D2)/D1 ≤ 0.06”, where a maximum value of a radius of the brim portion is D1, and a minimum value of the radius of the brim portion is D2, also satisfies “L2/L1 ≤ 0.30”, where a size of the center electrode along the axis direction is L1, and a size along the axis direction from boundary between the connecting portion and the leg portion to a center of gravity of the center electrode is L2.

Spark plug has center electrode having leg portion extending in an axis direction, brim portion located at rear end side with respect to the leg portion and protruding outwards in a radial direction with respect to the leg portion and connecting portion connecting the leg portion and the brim portion; insulator having penetration hole and supporting the center electrode; and seal member filling the penetration hole and fixing the brim portion and the insulator. The center electrode satisfies “(D1-D2)/D1 ≤ 0.06”, where a maximum value of a radius of the brim portion is D1, and a minimum value of the radius of the brim portion is D2, also satisfies “L2/L1 ≤ 0.30”, where a size of the center electrode along the axis direction is L1, and a size along the axis direction from boundary between the connecting portion and the leg portion to a center of gravity of the center electrode is L2.

A spark plug includes a center electrode, a tubular metallic shell which holds the center electrode therein in an insulated state, and a ground electrode which faces a forward end portion of the center electrode. The metallic shell has a penetration hole which penetrates the metallic shell in a direction intersecting an axial direction. The ground electrode has a press-fitted portion inserted into the penetration hole and fixed to the metallic shell, a projecting portion projecting toward an inner side of the metallic shell, and an insertion portion disposed between the press-fitted portion and the projecting portion, inserted into the penetration hole, and forming a space between the insertion portion and an inner surface of the penetration hole. An angle α formed between the inner surface of the penetration hole and an outer surface of the insertion portion falls within a range of 0°<α<90°.

A spark plug includes a center electrode, a tubular metallic shell which holds the center electrode therein in an insulated state, and a ground electrode which faces a forward end portion of the center electrode. The metallic shell has a penetration hole which penetrates the metallic shell in a direction intersecting an axial direction. The ground electrode has a press-fitted portion inserted into the penetration hole and fixed to the metallic shell, a projecting portion projecting toward an inner side of the metallic shell, and an insertion portion disposed between the press-fitted portion and the projecting portion, inserted into the penetration hole, and forming a space between the insertion portion and an inner surface of the penetration hole. An angle α formed between the inner surface of the penetration hole and an outer surface of the insertion portion falls within a range of 0°<α<90°.

An ignition plug includes: an insulator provided with a shaft hole formed therein; a metal fitting caulked to support the insulator from an outer periphery side thereof; and a talc material filled in an annular space provided between an outer peripheral surface of the insulator and an inner peripheral surface of the metal fitting. A first annular member and a second annular member are provided at an end portion of the annular space, the end portion being positioned in a center electrode side along the center axis in the annular space, the second annular member is in contact with the first annular member from an opposite side of the center electrode.

An ignition plug includes: an insulator provided with a shaft hole formed therein; a metal fitting caulked to support the insulator from an outer periphery side thereof; and a talc material filled in an annular space provided between an outer peripheral surface of the insulator and an inner peripheral surface of the metal fitting. A first annular member and a second annular member are provided at an end portion of the annular space, the end portion being positioned in a center electrode side along the center axis in the annular space, the second annular member is in contact with the first annular member from an opposite side of the center electrode.

A spark plug including a center electrode that has a large-diameter portion having an outside diameter that is largest in the center electrode. The large-diameter portion is retained at a rear facing surface in an axial hole of an insulator. A metal shell has a diameter increased portion on a rear end side with respect to the center electrode, the diameter increased portion having an inside diameter that is increased toward the rear end side. The insulator has a first portion which is a portion of the insulator in a region from a rear end of the rear facing surface to a front end of the diameter increased portion. The first portion has a thickness that is the largest in the region, and is disposed at least on an outer periphery of the large-diameter portion.

A spark plug including a center electrode that has a large-diameter portion having an outside diameter that is largest in the center electrode. The large-diameter portion is retained at a rear facing surface in an axial hole of an insulator. A metal shell has a diameter increased portion on a rear end side with respect to the center electrode, the diameter increased portion having an inside diameter that is increased toward the rear end side. The insulator has a first portion which is a portion of the insulator in a region from a rear end of the rear facing surface to a front end of the diameter increased portion. The first portion has a thickness that is the largest in the region, and is disposed at least on an outer periphery of the large-diameter portion.

A spark plug including a center electrode that has a large-diameter portion having an outside diameter that is largest in the center electrode. The large-diameter portion is retained at a rear facing surface in an axial hole of an insulator. A metal shell has a diameter increased portion on a rear end side with respect to the center electrode, the diameter increased portion having an inside diameter that is increased toward the rear end side. The insulator has a first portion which is a portion of the insulator in a region from a rear end of the rear facing surface to a front end of the diameter increased portion. The first portion has a thickness that is the largest in the region, and is disposed at least on an outer periphery of the large-diameter portion.

A spark plug including a center electrode that has a large-diameter portion having an outside diameter that is largest in the center electrode. The large-diameter portion is retained at a rear facing surface in an axial hole of an insulator. A metal shell has a diameter increased portion on a rear end side with respect to the center electrode, the diameter increased portion having an inside diameter that is increased toward the rear end side. The insulator has a first portion which is a portion of the insulator in a region from a rear end of the rear facing surface to a front end of the diameter increased portion. The first portion has a thickness that is the largest in the region, and is disposed at least on an outer periphery of the large-diameter portion.