A ground electrode (27) and a noble-metal chip (32) are joined via a weld portion (42) formed by fusing and mixing an Ni alloy and a Pt alloy. A plurality of acicular or rhizoid microcracks (51) are formed in the weld portion (42). As viewed on a section of the weld portion (42), the average length of the microcracks is 50 μm to 500 μm, and the average aspect ratio (shorter dimension/longer dimension) of the microcracks is 0.05 or less. At least one of the Ni alloy used to form the ground electrode (27) and the Pt alloy used to form the noble-metal chip (32) contains as an additive at least one of elements belonging to Groups 3A and 4A of the Periodic Table and oxides of these elements.