A spark plug manufacturing method for manufacturing a spark plug including: a rod-like center electrode extending in the direction of an axis; an approximately cylindrical tubular insulator having an axial hole extending in the direction of the axis, the center electrode being provided at a front end portion of the axial hole; an approximately cylindrical tubular metallic shell surrounding the insulator; and a ground electrode whose proximal end portion is joined to the metallic shell, and which is bent at an intermediate portion thereof so as to form a spark discharge gap between a distal end portion of the ground electrode and a front end portion of the center electrode, the method comprising a bending process of bending an unbent ground electrode toward the axis in a state in which the metallic shell with the unbent ground electrode joined thereto and the insulator in which the center electrode is provided are assembled together, wherein the bending process includes a bending step comprising disposing a curved surface forming means having a curved surface portion corresponding to a bent shape of the ground electrode between a side portion of the center electrode and the proximal end portion of the ground electrode, supporting the ground electrode on a side portion of the proximal end portion located on the side toward the center electrode by an inward deformation prevention means of a deformation prevention means so as to prevent deformation of the proximal end portion of the ground electrode toward the center electrode, supporting the ground electrode on a side portion of the proximal end portion opposite the center electrode by an outward deformation prevention means of a deformation prevention means so as to prevent deformation of the proximal end portion of the ground electrode toward a side opposite the center electrode, pressing the ground electrode against the curved surface portion from the side opposite the center electrode by use of bending means, whereby bending work is performed on the ground electrode. a rod-like center electrode extending in the direction of an axis; an approximately cylindrical tubular insulator having an axial hole extending in the direction of the axis, the center electrode being provided at a front end portion of the axial hole; an approximately cylindrical tubular metallic shell surrounding the insulator; and a ground electrode whose proximal end portion is joined to the metallic shell, and which is bent at an intermediate portion thereof so as to form a spark discharge gap between a distal end portion of the ground electrode and a front end portion of the center electrode, the method comprising a bending process of bending an unbent ground electrode toward the axis in a state in which the metallic shell with the unbent ground electrode joined thereto and the insulator in which the center electrode is provided are assembled together, wherein the bending process includes a bending step comprising disposing a curved surface forming means having a curved surface portion corresponding to a bent shape of the ground electrode between a side portion of the center electrode and the proximal end portion of the ground electrode, supporting the ground electrode on a side portion of the proximal end portion located on the side toward the center electrode by an inward deformation prevention means of a deformation prevention means so as to prevent deformation of the proximal end portion of the ground electrode toward the center electrode, supporting the ground electrode on a side portion of the proximal end portion opposite the center electrode by an outward deformation prevention means of a deformation prevention means so as to prevent deformation of the proximal end portion of the ground electrode toward a side opposite the center electrode,