<p>Disclosed is a technique that is capable of increasing the breaking strength of the insulator of a spark plug. In a cross section comprising the axis of the spark plug, the spark plug fulfills the relation 0.6 mm ? L when point (A) is the connecting point of an insulator support and an insulator body section that is formed further on the edge side than the insulator support, point (B) is the position that is further on the outer circumference side when comparing the innermost position from among locations at which the insulator support and a packing come into contact and the position at which a virtual line extending parallel to the axis from the innermost edge of the stepped section on a main fitting intersects with the insulator support, and (L) is the length of a channel along the surface of the insulator from point (A) to point (B).</p>
