A spark plug that allows a heat transfer member to be fixed to an insulator in a simple manner. The spark plug includes: a tubular insulator extending in an axial line direction from a front side to a rear side; and a tubular metal shell fixed to an outer circumference of the insulator and having an external thread formed on a part of an outer circumferential surface thereof, wherein the insulator has a groove formed on a portion, of an outer circumferential surface thereof, that overlaps the external thread of the metal shell in the axial line direction, a heat transfer member mounted in the groove is in contact with an inner circumferential surface of the metal shell, and a portion of the heat transfer member is disposed in the groove of the insulator.