A glow plug comprising: a tubular tube that extends along a direction of an axial line (CL1, CL2) and is closed at a front end portion thereof; and a heat-producing coil that is helically wound, arranged inside the tube substantially coaxially with the tube, and joined at an own one end to the front end portion of the tube, wherein in a specific cross-sectional region that is one of cross-sectional coil regions of the heat-producing coil observed in a vertical cross-sectional surface including a center axial line of the tube, a > b is satisfied, wherein a (mm) is a length of the specific cross-sectional region along the direction of the axial line (CL1, CL2), and b (mm) is a length of the specific cross-sectional region along a direction perpendicular to the axial line (CL1, CL2), characterized in that in said specific cross-sectional region, an inner appearance line, which is a line segment positioned on the side close to the center axial line (CL1, CL2) within an appearance line of the specific cross-sectional region, is in a form of a straight line or a curved line, the curved line being convex toward the center axial line so that R > a/2 is satisfied wherein R is a curvature radius R (mm), in a range positioned between both end points (P1, P3) among three points (P1, P2, P3) that divide the inner appearance line into quarters along the axial line direction, and wherein when L (mm) is defined as a distance along the direction perpendicular to the axial line (CL1, CL2) from a portion closest to the center axial line within the inner appearance line through a virtual straight line drawn at a position in the specific cross-sectional region in parallel with the axial line (CL1, CL2), the position allowing an area of the thus formed region closer to the inner appearance line to be 10% of an entire area of the specific cross-sectional region, 0.100 < L/b ≤ 0.144 is satisfied. a tubular tube that extends along a direction of an axial line (CL1, CL2) and is closed at a front end portion thereof; and a heat-producing coil that is helically wound, arranged inside the tube substantially coaxially with the tube, and joined at an own one end to the front end portion of the tube, wherein in a specific cross-sectional region that is one of cross-sectional coil regions of the heat-producing coil observed in a vertical cross-sectional surface including a center axial line of the tube, a > b is satisfied, wherein a (mm) is a length of the specific cross-sectional region along the direction of the axial line (CL1, CL2), and b (mm) is a length of the specific cross-sectional region along a direction perpendicular to the axial line (CL1, CL2), characterized in that in said specific cross-sectional region, an inner appearance line, which is a line segment positioned on the side close to the center axial line (CL1, CL2) within an appearance line of the specific cross-sectional region, is in a form of a straight line or a curved line, the curved line being convex toward the center axial line so that R > a/2 is satisfied wherein R is a curvature radius R (mm), in a range positioned between both end points (P1, P3) among three points (P1, P2, P3) that divide the inner appearance line into quarters along the axial line direction, and wherein when L (mm) is defined as a distance along the direction perpendicular to the axial line (CL1, CL2) from a portion closest to the center axial line within the inner appearance line through a virtual straight line drawn at a position in the specific cross-sectional region in parallel with the axial line (CL1, CL2), the position allowing an area of the thus formed region closer to the inner appearance line to be 10% of an entire area of the specific cross-sectional region, 0.100 < L/b ≤ 0.144 is satisfied.