A glow plug comprising: a heater having, in a forward end portion , a heat-generating resistor which generates heat through energization; a metallic shell assuming the form of a tube having an axial bore extending along a first axis (O) which is the axis of the metallic shell , and holding the heater directly or indirectly at a forward end portion ; a center shaft assuming a rodlike form, disposed in the axial bore of the metallic shell with a gap formed between the same and an inner circumferential surface of the axial bore , having one end portion connected to a rear end portion of the heater , and having the other end portion projecting from a rear end of the metallic shell ; and a contact member formed from an electrically insulative elastic member, assuming an annular form, inserted into the axial bore at a rear end portion of the axial bore , and disposed in such a manner as to be in contact with the inner circumferential surface of the axial bore and with the center shaft ; the glow plug being characterized in that : a contour of one of two cross sections resulting from cutting the contact member in a state prior to attachment to the glow plug by a plane which contains a second axis (P) which is the axis of the contact member has a first contour segment assuming the form of a curve extending along the second axis (P) and swelling radially outward with a radius R1 of curvature and a second contour segment assuming one of the following a) or b): a) the form of a straight through line extending along the second axis (P); b) the form of a curve extending along the second axis (P) and swelling radially inward with a radius R2 of curvature which satisfies a relational expression R1 < R2. a heater having, in a forward end portion , a heat-generating resistor which generates heat through energization; a metallic shell assuming the form of a tube having an axial bore extending along a first axis (O) which is the axis of the metallic shell , and holding the heater directly or indirectly at a forward end portion ; a center shaft assuming a rodlike form, disposed in the axial bore of the metallic shell with a gap formed between the same and an inner circumferential surface of the axial bore , having one end portion connected to a rear end portion of the heater , and having the other end portion projecting from a rear end of the metallic shell ; and a contact member formed from an electrically insulative elastic member, assuming an annular form, inserted into the axial bore at a rear end portion of the axial bore , and disposed in such a manner as to be in contact with the inner circumferential surface of the axial bore and with the center shaft ; the glow plug being a contour of one of two cross sections resulting from cutting the contact member in a state prior to attachment to the glow plug by a plane which contains a second axis (P) which is the axis of the contact member has a first contour segment assuming the form of a curve extending along the second axis (P) and swelling radially outward with a radius R1 of curvature and a second contour segment assuming one of the following a) or b): a) the form of a straight through line extending along the second axis (P); b) the form of a curve extending along the second axis (P) and swelling radially inward with a radius R2 of curvature which satisfies a relational expression R1 < R2. a) the form of a straight through line extending along the second axis (P); b) the form of a curve extending along the second axis (P) and swelling radially inward with a radius R2 of curvature which satisfies a relational expression R1 < R2. <EOS>