A spark plug, comprising: a rod-shaped center electrode (20, 320, 420) extending in an axis direction of the spark plug (100, 200, 300, 400, 500); an insulator having an axial hole formed in the axis direction so as to retain the center electrode (20, 320, 420) in the axial hole; a metal shell (50, 250, 450, 550) circumferentially surrounding and retaining the insulator; and a ground electrode (30, 230, 330, 430, 530) having a base end portion welded to the metal shell (50, 250, 450, 550) and a distal end portion (38, 338, 538) facing an axially front end portion of the center electrode (20, 320, 420) with a gap left between the distal end portion (38, 338, 538) of the ground electrode (30, 230, 330, 430, 530) and the front end portion of the center electrode (20, 320, 420), the ground electrode (30, 230, 330, 430, 530) including a surface layer (31, 231, 331, 431, 531) defining a surface of the ground electrode (30, 230, 330, 430, 530) and a core (32, 232, 332, 432, 532) located inside the surface layer (31, 231, 331, 431, 531) and having a higher thermal conductivity than that of the surface layer (31, 231, 331, 431, 531), the surface layer (31, 231, 331, 431, 531) having a thickness of 0.2 mm to 0.4 mm at a specific position that is located 1 mm from the base end portion in a direction toward the distal end portion (38, 338, 538) along an outer shape of the ground electrode (30, 230, 330, 430, 530), characterized in that: the spark plug is adapted to satisfy the following condition: W1 ≥ W2×1.55-(W3+0.25) where W1 (mm) is a width of the metal shell (50, 250, 450, 550) at a weld region (58, 258, 458) of the metal shell (50, 250, 450, 550) joined with the base end portion in a specific direction that extends perpendicular to the axis direction through a center line of the ground electrode (30, 230, 330, 430, 530); W2 (mm) is a thickness of the ground electrode (30, 230, 330, 430, 530) at the specific position in the specific direction; and W3 (mm) is a thickness of the surface layer (31, 231, 331, 431, 531) at the specific position in the specific direction. a rod-shaped center electrode (20, 320, 420) extending in an axis direction of the spark plug (100, 200, 300, 400, 500); an insulator having an axial hole formed in the axis direction so as to retain the center electrode (20, 320, 420) in the axial hole; a metal shell (50, 250, 450, 550) circumferentially surrounding and retaining the insulator; and a ground electrode (30, 230, 330, 430, 530) having a base end portion welded to the metal shell (50, 250, 450, 550) and a distal end portion (38, 338, 538) facing an axially front end portion of the center electrode (20, 320, 420) with a gap left between the distal end portion (38, 338, 538) of the ground electrode (30, 230, 330, 430, 530) and the front end portion of the center electrode (20, 320, 420), the ground electrode (30, 230, 330, 430, 530) including a surface layer (31, 231, 331, 431, 531) defining a surface of the ground electrode (30, 230, 330, 430, 530) and a core (32, 232, 332, 432, 532) located inside the surface layer (31, 231, 331, 431, 531) and having a higher thermal conductivity than that of the surface layer (31, 231, 331, 431, 531), the surface layer (31, 231, 331, 431, 531) having a thickness of 0.2 mm to 0.4 mm at a specific position that is located 1 mm from the base end portion in a direction toward the distal end portion (38, 338, 538) along an outer shape of the ground electrode (30, 230, 330, 430, 530), characterized in that: the spark plug is adapted to satisfy the following condition: W1 ≥ W2×1.55-(W3+0.25) where W1 (mm) is a width of the metal shell (50, 250, 450, 550) at a weld region (58, 258, 458) of the metal shell (50, 250, 450, 550) joined with the base end portion in a specific direction that extends perpendicular to the axis direction through a center line of the ground electrode (30, 230, 330, 430, 530); W2 (mm) is a thickness of the ground electrode (30, 230, 330, 430, 530) at the specific position in the specific direction; and W3 (mm) is a thickness of the surface layer (31, 231, 331, 431, 531) at the specific position in the specific direction. the spark plug is adapted to satisfy the following condition: W1 ≥ W2×1.55-(W3+0.25) where W1 (mm) is a width of the metal shell (50, 250, 450, 550) at a weld region (58, 258, 458) of the metal shell (50, 250, 450, 550) joined with the base end portion in a specific direction that extends perpendicular to the axis direction through a center line of the ground electrode (30, 230, 330, 430, 530); W2 (mm) is a thickness of the ground electrode (30, 230, 330, 430, 530) at the specific position in the specific direction; and W3 (mm) is a thickness of the surface layer (31, 231, 331, 431, 531) at the specific position in the specific direction.