A spark plug includes a cylindrical metallic shell having a shoulder portion, and a tubular insulator having a stepped portion which rests on the shoulder portion within the metallic shell. The insulator has a rear half made of aluminum oxide and a front half made of aluminum nitride, the rear half having a protrusion, while the front half having a housing, and tightly joining the protrusion within the housing. An overlapping portion of the protrusion and the housing is at least partly located in a plane which is perpendicular to a longitudinal axis of the center electrode, the plane passing through the shoulder portion of the metallic shell.