A spark plug (100, 1100) comprising: a rodlike center electrode (20, 1020) extending along an axial line; an insulator (10, 1010) having an axial hole extending along the axial line and holding the center electrode (20, 1020) in the axial hole in such a manner that the center electrode (20, 1020) protrudes axially forward from the axial hole; a metallic shell (50, 1050) holding the insulator (10, 1010) in such a manner as to circumferentially surround a portion of the insulator (10, 1010); and an annular seal member (8, 1008) for providing a seal between the insulator (10, 1010) and the metallic shell (50, 1050); the insulator (10, 1010) comprising a first portion (17, 1017), a second portion (13, 1013) located axially forward of the first portion (17, 1017) and being smaller in outside diameter than the first portion (17, 1017), and an insulator first-diameter-reducing-portion (15, 1015) whose outside diameter reduces axially forward and which connects the first portion (17, 1017) and the second portion (13, 1013), the metallic shell (50, 1050) comprising a protrusion (56, 1060) protruding radially inward, and the protrusion (56, 1060) comprising a metallic shell diameter-reducing-portion (56, 1062) whose inside diameter reduces axially forward, and the seal member (8, 1008) being disposed between the insulator first-diameter-reducing-portion (15, 1015) and the metallic shell diameter-reducing-portion (56, 1062) at such a position (PP, -) as to cross an extension line (-, EL1) formed by imaginarily extending an outer surface of the first portion (17, 1017) in an axially forward direction, wherein a first angle θ21 and a second angle θ22 satisfy a relational expression θ21 > θ22 where, in or on a section which contains the axial line, the first angle θ21 is an acute angle between a straight line orthogonal to the axial line and an outline of the metallic shell diameter-reducing-portion (56, 1062), and the second angle θ22 is an acute angle between a straight line orthogonal to the axial line and an outline of the insulator first-diameter-reducing-portion (15, 1015); wherein the first angle θ21 and the second angle θ22 satisfy a relational expression θ21 - θ22 ≤ 7° characterized in that the second angle θ22 satisfies a relational expression θ22 ≥ 30°. a rodlike center electrode (20, 1020) extending along an axial line; an insulator (10, 1010) having an axial hole extending along the axial line and holding the center electrode (20, 1020) in the axial hole in such a manner that the center electrode (20, 1020) protrudes axially forward from the axial hole; a metallic shell (50, 1050) holding the insulator (10, 1010) in such a manner as to circumferentially surround a portion of the insulator (10, 1010); and an annular seal member (8, 1008) for providing a seal between the insulator (10, 1010) and the metallic shell (50, 1050); the insulator (10, 1010) comprising a first portion (17, 1017), a second portion (13, 1013) located axially forward of the first portion (17, 1017) and being smaller in outside diameter than the first portion (17, 1017), and an insulator first-diameter-reducing-portion (15, 1015) whose outside diameter reduces axially forward and which connects the first portion (17, 1017) and the second portion (13, 1013), the metallic shell (50, 1050) comprising a protrusion (56, 1060) protruding radially inward, and the protrusion (56, 1060) comprising a metallic shell diameter-reducing-portion (56, 1062) whose inside diameter reduces axially forward, and the seal member (8, 1008) being disposed between the insulator first-diameter-reducing-portion (15, 1015) and the metallic shell diameter-reducing-portion (56, 1062) at such a position (PP, -) as to cross an extension line (-, EL1) formed by imaginarily extending an outer surface of the first portion (17, 1017) in an axially forward direction, wherein a first angle θ21 and a second angle θ22 satisfy a relational expression θ21 > θ22 where, in or on a section which contains the axial line, the first angle θ21 is an acute angle between a straight line orthogonal to the axial line and an outline of the metallic shell diameter-reducing-portion (56, 1062), and the second angle θ22 is an acute angle between a straight line orthogonal to the axial line and an outline of the insulator first-diameter-reducing-portion (15, 1015); wherein the first angle θ21 and the second angle θ22 satisfy a relational expression θ21 - θ22 ≤ 7° characterized in that the second angle θ22 satisfies a relational expression θ22 ≥ 30°.