[Objective] To provide a spark plug which is configured such that an insulation member, from which a center electrode projects, is pressed frontward and held in a metallic shell, and is fixed by means of crimping the rear end of the metallic shell, and which can prevent a drop in gas tightness between the metallic shell and the insulation member due to a difference in thermal expansion therebetween.[Means for Solution] In a spark plug in which a mating shaft portion (10) of an insulation member (1) is loose-fitted into a mating hole portion (30) of a metallic shell (21), a filler (41) for maintaining gas tightness is charged between the outer circumferential surface of the mating shaft portion (10) and the inner circumferential surface of the mating hole portion (30). Despite the thermal expansion difference, the gas tightness is maintained, because the filler (41) for maintaining gas tightness is charged between the inner and outer circumferential surfaces.