A spark plug comprising: a pipe-shaped insulator having an axial hole that passes through the insulator in an axial direction; a center electrode projecting from a distal end of the insulator; a metal shell covering a peripheral portion of the insulator; and a ground electrode whose base end portion is joined to a distal end portion of the metal shell, the ground electrode having a bent portion that is bent such that a distal end portion of the ground electrode is disposed and spaced from a distal end portion of the center electrode, wherein a hardness distribution is obtained by cutting the ground electrode from a distal end to a base end of the ground electrode at a cutting plane including an axial line of the spark plug and passing through a center of the ground electrode, and then measuring hardness of the ground electrode at a plurality of positions disposed with a distance from the base end of the ground electrode along a center line of the cutting plane of the ground electrode, the distance increasing in increments of 0.1 mm, "n" is a natural number, a portion of the ground electrode from a position at a distance of 0.1 mm from the base end along the center line to the distal end is categorizable into a high hardness portion and a low hardness portion using the hardness distribution, the high hardness portion being a portion from the position at a distance of 0.1 mm from the base end along the center line to a position at a distance of 0.1 × n (mm) from the base end along the center line, the low hardness portion being a portion from a position at a distance of 0.1 × (n + 1) (mm) from the base end along the center line to the distal end, the low hardness portion includes a portion that has a largest curvature in the ground electrode, and a highest hardness of the low hardness portion is lower than a lowest hardness of the high hardness portion. a pipe-shaped insulator having an axial hole that passes through the insulator in an axial direction; a center electrode projecting from a distal end of the insulator; a metal shell covering a peripheral portion of the insulator; and a ground electrode whose base end portion is joined to a distal end portion of the metal shell, the ground electrode having a bent portion that is bent such that a distal end portion of the ground electrode is disposed and spaced from a distal end portion of the center electrode, wherein a hardness distribution is obtained by cutting the ground electrode from a distal end to a base end of the ground electrode at a cutting plane including an axial line of the spark plug and passing through a center of the ground electrode, and then measuring hardness of the ground electrode at a plurality of positions disposed with a distance from the base end of the ground electrode along a center line of the cutting plane of the ground electrode, the distance increasing in increments of 0.1 mm, "n" is a natural number, a portion of the ground electrode from a position at a distance of 0.1 mm from the base end along the center line to the distal end is categorizable into a high hardness portion and a low hardness portion using the hardness distribution, the high hardness portion being a portion from the position at a distance of 0.1 mm from the base end along the center line to a position at a distance of 0.1 × n (mm) from the base end along the center line, the low hardness portion being a portion from a position at a distance of 0.1 × (n + 1) (mm) from the base end along the center line to the distal end, the low hardness portion includes a portion that has a largest curvature in the ground electrode, and a highest hardness of the low hardness portion is lower than a lowest hardness of the high hardness portion.