Patent ID: 8810119

Claim:
A spark plug for an internal combustion engine, comprising: a tubular housing; a tubular insulator which is held inside the housing; a center electrode which is held inside the insulator so that an end portion thereof projects; and an earth electrode which has an opposing portion opposed to the end portion of the center electrode, a spark discharge gap being formed between the opposing portion and the end portion, wherein the end portion is provided with an end projecting portion projecting toward the opposing portion, the opposing portion is provided with an opposing projecting portion which projects toward the end portion and is opposed to the end projecting portion, the end projecting portion and the opposing projecting portion are provided with non-projection direction opposing surfaces which are parallel to each other and are opposed to each other with a minimum distance being interposed therebetween and in a direction other than a projection direction thereof, when defining an opposing area of a portion which includes the non-projection direction opposing surfaces and where the end projecting portion and the opposing projecting portion are opposed to each other with the minimum distance being interposed therebetween, as a first opposing area, and defining an opposing area obtained when a plane of the end projecting portion and a plane of the opposing projecting portion, which are orthogonal to the projection direction, are opposed to each other in the projection direction, as a second opposing area, the first opposing area is larger than the second opposing area, and one of the end projecting portion of the center electrode and the opposing projecting portion of the earth electrode is provided with a projection portion, the whole outer periphery of at least part of which is covered with the other of the end projecting portion and the opposing projecting portion and which projects toward the other of the end projecting portion and the opposing projecting portion.