Manufacturing method of main metal fitting for spark plug and manufacturing method of spark plug

A method of manufacturing a metallic shell for an ignition plug, the metallic shell having a tubular shape, extending in a direction of an axial line, and having a threaded portion on its outer circumferential surface to be engaged with a mounting hole of a combustion apparatus, the method comprising:

• a metallic shell tubular intermediate forming step of forming a metallic shell tubular intermediate by providing a tubular portion is a metallic shell intermediate that is to become the metallic shell for an ignition plug, and
• a rolling step of performing rolling on the metallic shell tubular intermediate using rolling dies to form the thread portion,
• wherein
• the metallic shell tubular intermediate forming step comprises the steps of:
• forming a first tubular portion at an end portion of the metallic shell intermediate, and
• forming a second tubular portion at at least a portion of that region of the metallic shell intermediate which differs from the first tubular portion;
• and
• in the rolling step, a bearing member is inserted into the metallic shell tubular intermediate for nipping the metallic shell tubular intermediate in cooperation with working surfaces of the rolling dies, and rolling is performed simultaneously on at least the first tubular portion and the second tubular portion such that a post-rolling radial eccentricity between a center axis of the first tubular portion and a center axis of the second tubular portion becomes smaller than a pre-rolling radial eccentricity between the center axis of the first tubular portion and the center axis of the second tubular portion.