A glow plug having a leading front end and an opposing rear end, and including a combustion pressure detection sensor, the glow plug comprising: a housing having a cylindrical shape which houses a heater having a pillar shape, a leading end of the heater protruding from a leading end of the housing; and a sensor that detects the combustion pressure based on displacement of the heater by action of the combustion pressure; and an annular gap between an inner peripheral surface of a leading end part of a predetermined range from the leading end of the housing toward a rear part thereof and an outer peripheral surface of the heater, sand a deformable seal member allowing for displacement of the heater is provided in the annular gap so as to divide the annular gap into front and rear sides, wherein, in the outer peripheral surface of the heater, in a part facing the inner peripheral surface of the leading end part of the housing interposed in the annular gap, a convex portion protruding outward in a radial direction is formed so as to maintain a gap with the inner peripheral surface, wherein the leading end of the convex portion is formed so as to be located further to the front than the leading end of the housing, and a rear end of the convex portion is located behind or ahead of the inner peripheral surface, characterized in that in the seal member, an inner periphery edge portion at the outer peripheral surface side of the heater has a cylindrical portion bent and extended frontwards, the cylindrical portion being fixed to the outer peripheral surface of the heater, in that in a radial direction, an outermost peripheral portion of the convex portion is located outside the outer peripheral portion of a leading end of the cylindrical portion of the seal member, and in that the convex portion is formed of a cylindrical pipe externally fitted to the heater and is fixed by welding. a housing having a cylindrical shape which houses a heater having a pillar shape, a leading end of the heater protruding from a leading end of the housing; and a sensor that detects the combustion pressure based on displacement of the heater by action of the combustion pressure; and an annular gap between an inner peripheral surface of a leading end part of a predetermined range from the leading end of the housing toward a rear part thereof and an outer peripheral surface of the heater, sand a deformable seal member allowing for displacement of the heater is provided in the annular gap so as to divide the annular gap into front and rear sides, wherein, in the outer peripheral surface of the heater, in a part facing the inner peripheral surface of the leading end part of the housing interposed in the annular gap, a convex portion protruding outward in a radial direction is formed so as to maintain a gap with the inner peripheral surface, wherein the leading end of the convex portion is formed so as to be located further to the front than the leading end of the housing, and a rear end of the convex portion is located behind or ahead of the inner peripheral surface, characterized in that in the seal member, an inner periphery edge portion at the outer peripheral surface side of the heater has a cylindrical portion bent and extended frontwards, the cylindrical portion being fixed to the outer peripheral surface of the heater, in that in a radial direction, an outermost peripheral portion of the convex portion is located outside the outer peripheral portion of a leading end of the cylindrical portion of the seal member, and in that the convex portion is formed of a cylindrical pipe externally fitted to the heater and is fixed by welding.