A spark plug comprising: an insulator having an axial hole extending in a direction of an axis; a center electrode held at one end of the axial hole; a metallic terminal which has an accommodated portion accommodated in the axial hole and which is held at the other end of the insulator; and a connecting portion which electrically connects the center electrode and the metallic terminal within the axial hole, the spark plug being characterized in that the side of the axial hole where the metallic terminal is held is defined as a rear end side with respect to the direction of the axis; a connection member which constitutes the connecting portion is provided between an outer circumferential surface of a forward end portion of the accommodated portion and an inner circumferential surface of the insulator; and in sectional images orthogonal to the axis which are captured at intervals of 0.5 mm from a rear end of the accommodated portion toward a forward end thereof up to a rear end position where the connection member is present, on a sectional image S n at the n-th section (n is a natural number) counted from the rear end of the accommodated portion, a center of the accommodated portion is defined as a point a n, a center of the axial hole is defined as a point O n, a line which passes through the points a n and O n is defined as a line L n, a diameter of the axial hole is represented by 2R n, a diameter of the accommodated portion is represented by 2r n, a distance between the point a n and the point O n is represented by H aon, and the point a n which satisfies a relation H aon ≥ 0.8 (R n - r n) is defined as a point A n, on the sectional image S 1, a line which is obtained by rotating a line L 1 by 45° about the point O 1 is defined as a line L 1+, a line which is obtained by rotating the line L 1 by -45° about the point O 1 is defined as a line L 1-, four areas which are surrounded by a plane X + containing the line L 1+ and the axis, a plane X - containing the line L 1- and the axis, and the inner circumferential surface of the insulator are identified by T 1, T 2, T 3, and T 4, respectively, and when successive points A m to A (m+k) (m and k are natural numbers) are present in a specific area selected from the areas T 1, T 2, T 3, and T 4, a group containing these points A m to A (m+k) is defined as a group B m,y, where y is a natural number and means that the group is the y-th group counted from the rear end side of the accommodated portion, wherein the maximum value of y is at least 3, and at least two of the groups B m,y are present in two areas which are selected from the areas T 1 to T 4 and which are located at symmetrical positions. an insulator having an axial hole extending in a direction of an axis; a center electrode held at one end of the axial hole; a metallic terminal which has an accommodated portion accommodated in the axial hole and which is held at the other end of the insulator; and a connecting portion which electrically connects the center electrode and the metallic terminal within the axial hole, the spark plug being characterized in that the side of the axial hole where the metallic terminal is held is defined as a rear end side with respect to the direction of the axis; a connection member which constitutes the connecting portion is provided between an outer circumferential surface of a forward end portion of the accommodated portion and an inner circumferential surface of the insulator; and in sectional images orthogonal to the axis which are captured at intervals of 0.5 mm from a rear end of the accommodated portion toward a forward end thereof up to a rear end position where the connection member is present, on a sectional image S n at the n-th section (n is a natural number) counted from the rear end of the accommodated portion, a center of the accommodated portion is defined as a point a n, a center of the axial hole is defined as a point O n, a line which passes through the points a n and O n is defined as a line L n, a diameter of the axial hole is represented by 2R n, a diameter of the accommodated portion is represented by 2r n, a distance between the point a n and the point O n is represented by H aon, and the point a n which satisfies a relation H aon ≥ 0.8 (R n - r n) is defined as a point A n, on the sectional image S 1, a line which is obtained by rotating a line L 1 by 45° about the point O 1 is defined as a line L 1+, a line which is obtained by rotating the line L 1 by -45° about the point O 1 is defined as a line L 1-, four areas which are surrounded by a plane X + containing the line L 1+ and the axis, a plane X - containing the line L 1- and the axis, and the inner circumferential surface of the insulator are identified by T 1, T 2, T 3, and T 4, respectively, and when successive points A m to A (m+k) (m and k are natural numbers) are present in a specific area selected from the areas T 1, T 2, T 3, and T 4, a group containing these points A m to A (m+k) is defined as a group B m,y, where y is a natural number and means that the group is the y-th group counted from the rear end side of the accommodated portion, wherein the maximum value of y is at least 3, and at least two of the groups B m,y are present in two areas which are selected from the areas T 1 to T 4 and which are located at symmetrical positions.