As shown in FIG. 11, for example, the spark plug for an internal combustion engine such as an automotive engine is provided with a center electrode 81, an insulator 82 disposed on the outer side of the center electrode 81, a cylindrical metal shell 83 disposed on the outer side of the insulator 82, and a ground electrode 84 bonded at its rear end portion to the leading end portion of the metal shell 83. The ground electrode 84 is made by bending a metallic rod having a substantially square section toward the center at its longitudinally intermediate position. Moreover, the ground electrode 84 is arranged such that its leading end portion has its inner side face confronting the leading end face of the center electrode 81. As a result, a spark discharge gap is formed between the leading end portion of the center electrode 81 and the inner face of the leading end portion of the ground electrode 84.
A not-shown threaded portion is formed in the outer circumference of the metal shell 83. The spark plug is mounted in the cylinder head of the engine by fastening it at that threaded portion. In case the ground electrode 84 of the spark plug mounted and a mixture, as indicated by arrows, take the positional relation, as shown, in which the mixture impinges directly upon the back of the ground electrode 84, the ground electrode 84 may obstruct the inflow of the mixture to the spark discharge gap of the mixture. As a result, the mixture may find it difficult to reach the spark discharge gap thereby to deteriorate the ignitability.
In the spark plug of the type having two or more ground electrode, on the other hand, there is a technique (as referred to JP-A-11-121142, for example), in which the individual ground electrodes are formed into a cylindrical shape having a substantially circular section. With this substantially circular section, the mixture goes around to the inner side of the ground electrodes so that it easily reaches the spark discharge gap, even in the positional relation where the mixture impinges directly upon the backs of the ground electrodes.