At least one of a center electrode tip and a ground electrode tip is made of an iridium alloy. A discharge gap, formed between the center electrode tip and the ground electrode tip, is less than 1.1 mm. And, cross sections of the center electrode tip and the ground electrode tip are equal to or smaller than 0.95 mm2 in a spherical region where a distance from a midpoint of the discharge gap is within 0.6 mm.