A spark plug comprising a shaft-like insulation member made of ceramic and having a center electrode projecting from a front end thereof, and a metallic shell fixing the insulation member in a surrounding manner and having a ground electrode provided at a front end thereof, the insulation member having a front end shaft portion located toward a front end thereof and formed in such a manner as to form an annular insulation space between the front end shaft portion and an inner circumferential surface of the metallic shell, and a mating shaft portion located rearward of the front end shaft portion, having a diameter greater than that of the front end shaft portion, and disposed in a mating hole portion of the metallic shell, the insulation member being inserted into the metallic shell from a rear end of the metallic shell, a frontward movement of the insulation member in the course of insertion being checked at a predetermined position by stopper means, and the insulation member being fixed in the metallic shell in a pressed-frontward condition by means of a crimp portion provided at the rear end of the metallic shell being bent toward an axis and pressed frontward, wherein the stopper means comprises an annular butt portion of the insulation member located rearward of the mating shaft portion, having a diameter greater than that of the mating shaft portion, and assuming the form of a surface facing frontward, and an annular support ledge of the metallic shell located rearward of the mating hole portion, having a diameter greater than that of the mating hole portion, and assuming the form of a surface facing rearward and the butt portion butts directly or indirectly against the support ledge to thereby check frontward movement of the insulation member, characterized in that a filler for maintaining gas tightness is provided in a filling manner between an outer circumferential surface of the mating shaft portion and an inner circumferential surface of the mating hole portion; wherein the filler is a heat-resistant adhesive; wherein the heat-resistant adhesive bonds the inner circumferential surface of the mating hole portion and the outer circumferential surface of the mating shaft portion together, thus maintaining gas tightness. the insulation member having a front end shaft portion located toward a front end thereof and formed in such a manner as to form an annular insulation space between the front end shaft portion and an inner circumferential surface of the metallic shell, and a mating shaft portion located rearward of the front end shaft portion, having a diameter greater than that of the front end shaft portion, and disposed in a mating hole portion of the metallic shell, the insulation member being inserted into the metallic shell from a rear end of the metallic shell, a frontward movement of the insulation member in the course of insertion being checked at a predetermined position by stopper means, and the insulation member being fixed in the metallic shell in a pressed-frontward condition by means of a crimp portion provided at the rear end of the metallic shell being bent toward an axis and pressed frontward, characterized in that a filler for maintaining gas tightness is provided in a filling manner between an outer circumferential surface of the mating shaft portion and an inner circumferential surface of the mating hole portion; wherein the filler is a heat-resistant adhesive; wherein the heat-resistant adhesive bonds the inner circumferential surface of the mating hole portion and the outer circumferential surface of the mating shaft portion together, thus maintaining gas tightness.