Spark plug designed to enhance strength of joint of noble metal member to ground electrode

A spark plug comprising:

• a metal shell;
• a center electrode retained within said metal shell to be insulated electrically from said metal shell;
• a ground electrode which is made of a Ni-based alloy and retained by said metal shell, said ground electrode having a center electrode-facing surface opposed to said center electrode;
• an Ir-alloy member which is welded to the center electrode-facing surface of said ground electrode and opposed to said center electrode through a spark gap;
• and
• fused portions which are formed by welding of said Ir-alloy member to said ground electrode to establish joints between said ground electrode and said Ir-alloy member and include a melted mixture of materials of said ground electrode and said Ir-alloy member, each of the fused portions containing less than 18% by weight of Ir in a range defined inside an imaginary plane coinciding with an outer peripheral wall of said Ir-alloy member before welded to said ground electrode,
• wherein the center electrode-facing surface of said ground electrode has formed therein a recess in which said Ir-alloy member is embedded, and wherein each of said fused portions extends from an outer side surface of said ground electrode, between the center electrode-facing surface and a surface opposed to the center electrode-facing surface, to inside a portion of said Ir-alloy member located within the recess.