Patent ID: 8398447

Claim:
A method for manufacturing the spark plug for an internal-combustion engine having an attachment fitting which provides a screw part to its outer circumference, an insulator held by the attachment fitting so that the insulator tip part may project, a center electrode held by the insulator so that the electrode tip part may project from the insulator tip part, and an earth electrode which forms a spark discharge gap between the center electrode and the earth electrode, wherein: the earth electrode has a convex part formed by projecting toward the center electrode a part of the opposed surface, which faces the center electrode, of the earth electrode and a concave part formed toward the opposed surface from the earth electrode's back surface which is the reverse side of the opposed surface of the earth electrode while the earth electrode is fixed to the attachment fitting; the convex part is disposed so that the extension of a shaft center of the convex part may pass through the area in which the concave part is formed; and a relation of S 1 >=s is realized when an area of an opening of the concave part is set to S 1 and an average cross-section area of a cross section of the convex part perpendicular to an axial direction of the spark plug is set to s, wherein the method comprises steps of: laying the approximately flat earth electrode on a metallic mold which has a cavity for convex part for shaping the convex part in the state where the cavity for convex part opposes the opposed surface; forming the concave part by pressing a part of the earth electrode's back surface with a pressing jig for forming the concave part; and forming the convex part by pushing out a part of the earth electrode to the cavity for convex part.