SPARK PLUG

[Purpose] To secure the joint strength between a ground electrode and a metal shell even when a spark plug is reduced in diameter. [Solution] A spark plug 100 includes a center electrode 20, a ground electrode 30 formed of a metal material containing 95% or more of nickel and a substantially cylindrical metal shell 50 having a front end face 57 to which one end of the ground electrode 30 is welded. In the spark plug 100, the conditions: 0.15 mm ‰¤ BD ‰¤ 0.40 mm; and (EW2-EW1)/EW1 ‰¥ 0.1 are satisfied where BD is a depth from the front end face of the metal shell 50 to a portion of the ground electrode 30 embedded most deeply in the metal shell 50; EW1 is a width of a portion of the ground electrode 30 located closest to a portion of the ground electrode 30 deformed by the welding; and EW2 is a width of the portion of the ground electrode 30 deformed by the welding at the front end face 57 of the metal shell 50.