A spark plug for an internal combustion engine is provided which includes a metal shell, a porcelain insulator, a center electrode, and a ground electrode. The center electrode is retained in the porcelain insulator to define a spark gap between itself and the ground electrode. The porcelain insulator has a nose made up of an upright portion and a tapered portion continuing from the upright portion toward a top end thereof. The tapered portion has a diameter decreasing toward the top end of the porcelain insulator. The upright portion has an outer wall extending substantially parallel to an inner wall of the metal shell, thereby inducing the formation of side sparks between the tapered portion and the metal shell before the insulation resistance between the center electrode and the metal shell drops, thereby giving a signal indicative of such an event to an operator.