To provide technique capable of easily manufacturing a center shaft.

A glow plug comprises: a metal fitting having a through hole extending along an axial direction; the center shaft inserted in the through hole and arranged with a gap left between the outer peripheral surface of the center shaft and the inner peripheral surface of the through hole; and an O-ring sealing the gap between the center shaft and the through hole by contacting with both the outer peripheral surface of the center shaft and the inner peripheral surface of the through hole. The maximum height of surface roughness along the circumferential direction of the outer peripheral surface of the center shaft at a portion contacting with the O-ring is 12.5 μm or less.