Problem is to enhance the welding strength when a projecting shape section is resistance-welded to a ground electrode. A ground electrode 30 includes a ground electrode base material 35 and a projecting shape section 36. The projecting shape section 36 is connected by resistance welding to an opposite surface 32 of the ground electrode 30 so as to be opposite and project towards the leading end of a center electrode 20. The ground electrode base material 35 and the projecting shape section 36 are formed from a material that is composed of the same metal (for example, nickel) as a main component and have a relation of formulas (1) and (2) described below. In formula (1), the specific resistance of the ground electrode base material 35 is R (µ©cm) and the specific resistance of the projecting shape section 36 is S (µ©cm). The specific resistance R > the specific resistance S...(formula 1), the specific resistance R - specific resistance S‰¥ 20...(formula 2). Accordingly, fusion of the ground electrode base material 35 that has a larger volume than that of the projecting shape section 36 can be expedited and the welding strength can be enhanced.