A spark plug has a cylindrical housing case, a cylindrical glass insulator, a center electrode, and an earth electrode fixed to the cylindrical housing case. The earth electrode has a projected part which is projected toward the center electrode. The projected part has a facing surface which faces the center electrode and is most close to the center electrode in a spark discharge gap. This gap is formed between the center electrode and the earth electrode. The facing surface is covered with a plating layer. A base material, with which the earth electrode is made, is exposed on a side surface of the projected part around the facing surface of the projected part. The base material of the earth electrode is made of Ni alloy containing not less than 90 wt % Ni, and preferably, Ni within a range of 90 wt % to 98 wt %.