Glow plug

A glow plug comprising:

• a tubular housing extending in a direction of an axis;
• and
• a rod-shaped heater whose forward end portion projects from a forward end of the housing and whose rear end portion is held inside the housing, characterized in that
• the housing has a forward end portion having an outer peripheral surface whose diameter increases toward the rear end side with respect to the axial direction, the forward end portion coming into contact with a seat surface formed on an internal combustion engine when the glow plug is attached to a through-hole provided in the internal combustion engine;
• the outer peripheral surface of the forward end portion includes a plurality of contact surfaces which differ in imaginary line angle from one another, the imaginary line angle of each contact surface being an angle which is formed, as viewed on a cross section including the axis, between two straight imaginary lines connecting inflection points of the contact surface at opposite ends of the contact surface;
• and
• at least one of the plurality of contact surfaces is a curved surface which bulges outward,
• wherein in the plurality of contact surfaces, an imaginary line angle of contact surface provided on the most forward end side with respect to the axial direction is 130° to 110°, and an imaginary line angle of contact surface provided on the most rear end side with respect to the axial direction is 70° to 50°.