A glow plug (1) provided with heat-producing coil (9) inside tube (7). When a vertical cross-section including a center axial line CL2 is observed, a specific cross-sectional region (21) satisfies a>b, where a (mm) is a length along a direction of an axial line CL1, and b (mm) is a length along a direction perpendicular to the axial line CL1. In addition, an inner appearance line (221) of the specific cross-sectional region 21 satisfies R>a/2 in a range positioned between predetermined points P1 and P3, wherein R (mm) is a curvature radius. Moreover, 0.100
