A spark plug includes a center electrode, an insulator, a metallic shell, and a seal member for providing a seal between the insulator and the metallic shell. The insulator includes a first portion, a second portion located axially forward of the first portion and being smaller than the first portion, and an insulator first-diameter-reducing-portion whose outside diameter reduces forward and which connects the first portion and the second portion. The metallic shell includes a protrusion that includes a metallic shell diameter-reducing-portion whose inside diameter reduces forward. The seal member is disposed between the insulator first-diameter-reducing-portion and the metallic shell diameter-reducing-portion. A relationship θ21>θ22 is satisfied, where the angle θ21 is an angle between a straight line orthogonal to the axial line and the outline of the metallic shell diameter-reducing-portion, and the angle θ22 is an angle between the straight line and an outline of the insulator first-diameter-reducing-portion.