A spark plug comprising: a tubular insulator having an axial hole extending in a direction of an axial line; a center electrode inserted into a forward portion of the axial hole; and a tubular metallic shell provided around the insulator and having a protrusion projecting inwardly in a radial direction, wherein the protrusion has a diameter-decreasing portion whose diameter decreases toward the forward end of the metallic shell; the insulator has, on an outer wall thereof, an engagement portion whose diameter decreases toward the forward end of the insulator; and the engagement portion seats on the diameter-decreasing portion via an annular plate packing, wherein in or on a longitudinal cross section including the axial line: a relation of θs > θp is satisfied, wherein θp represents an acute angle (°) between a straight line orthogonal to the axial line and the contour of the engagement portion, and θs represents an acute angle (°) between a straight line orthogonal to the axial line and the contour of the diameter-decreasing portion; the plate packing is disposed so as to include a first line segment extending, in the direction of the axial line, between the rear end of the engagement portion and the diameter-decreasing portion; and a relation of Hvo > Hvi is satisfied, wherein Hvo represents the Vickers hardness (Hv) of the plate packing at the midpoint of the first line segment, and Hvi represents the Vickers hardness (Hv) of the plate packing at the midpoint of a second line segment extending, in the direction of the axial line, between the engagement portion and the forward end of the diameter-decreasing portion which is in contact with the plate packing, characterized in that a relation of 1.03 ≤ Hvo/Hvi ≤ 1.25 is satisfied. a tubular insulator having an axial hole extending in a direction of an axial line; a center electrode inserted into a forward portion of the axial hole; and a tubular metallic shell provided around the insulator and having a protrusion projecting inwardly in a radial direction, wherein the protrusion has a diameter-decreasing portion whose diameter decreases toward the forward end of the metallic shell; the insulator has, on an outer wall thereof, an engagement portion whose diameter decreases toward the forward end of the insulator; and the engagement portion seats on the diameter-decreasing portion via an annular plate packing, wherein in or on a longitudinal cross section including the axial line: a relation of θs > θp is satisfied, wherein θp represents an acute angle (°) between a straight line orthogonal to the axial line and the contour of the engagement portion, and θs represents an acute angle (°) between a straight line orthogonal to the axial line and the contour of the diameter-decreasing portion; the plate packing is disposed so as to include a first line segment extending, in the direction of the axial line, between the rear end of the engagement portion and the diameter-decreasing portion; and a relation of Hvo > Hvi is satisfied, wherein Hvo represents the Vickers hardness (Hv) of the plate packing at the midpoint of the first line segment, and Hvi represents the Vickers hardness (Hv) of the plate packing at the midpoint of a second line segment extending, in the direction of the axial line, between the engagement portion and the forward end of the diameter-decreasing portion which is in contact with the plate packing, characterized in that a relation of 1.03 ≤ Hvo/Hvi ≤ 1.25 is satisfied. a relation of θs > θp is satisfied, wherein θp represents an acute angle (°) between a straight line orthogonal to the axial line and the contour of the engagement portion, and θs represents an acute angle (°) between a straight line orthogonal to the axial line and the contour of the diameter-decreasing portion; the plate packing is disposed so as to include a first line segment extending, in the direction of the axial line, between the rear end of the engagement portion and the diameter-decreasing portion; and a relation of Hvo > Hvi is satisfied, wherein Hvo represents the Vickers hardness (Hv) of the plate packing at the midpoint of the first line segment, and Hvi represents the Vickers hardness (Hv) of the plate packing at the midpoint of a second line segment extending, in the direction of the axial line, between the engagement portion and the forward end of the diameter-decreasing portion which is in contact with the plate packing, characterized in that a relation of 1.03 ≤ Hvo/Hvi ≤ 1.25 is satisfied.