In a spark plug having a nickel-based electrode whose front end has a firing tip made from a ruthenium- or iridium-based metal in which an oxide of a rare earth metal group is dispersed, the firing tip is welded to the electrode by a solidified alloy layer having a component of the electrode and a component of the firing tip. The firing tip contains the oxide of the rare earth metal group in a range of 5.about.15% by volume (V), and an average grain size (D) of the oxide is in a range of 0.05.about.3.0 .mu.m with a quantitative relationship as D.ltoreq.-0.34V+5.1.