A spark plug (100, 100b) comprising: a tubular insulator (10, 10b) having an axial hole (12, 12b) extending therethrough along an axial line; a center electrode disposed at an end of the axial hole (12, 12b) on a forward end side; and a rod-shaped ground electrode forming a gap (g) between the ground electrode and a portion of the center electrode on the forward end side, wherein the center electrode includes a shaft portion, a tip portion joined to a forward end portion of the shaft portion, and a joint portion joining the shaft portion and the tip portion together; an end of the joint portion on the forward end side is located on a rear end side (D1r) in the direction of the axial line in relation to an inner edge of the insulator (10, 10b), which is a forward-end-side edge of an inner circumferential surface of a small diameter portion (10r, 14r) of the insulator (10, 10b), the small diameter portion being a part of an accommodation portion of the insulator (10, 10b) for accommodating the tip portion, which part has the smallest inner diameter in the accommodation portion; characterized in that when a position separated from an edge of an end surface of the center electrode on the forward end side by 5 mm in a direction orthogonal to the axial line is defined as a first position, a position on the inner edge of the insulator (10, 10b) is defined as a second position, a position of intersection between the surface of the center electrode and a straight line (Lpr, Lprb) is defined as a third position, as viewed on a cross section including the axial line, the straight line (Lpr, Lprb) passing through the first position and being tangent, at one position, to a forward-end-side portion (R, 10p) of a contour of the insulator (10, 10b) on a side toward the first position with respect to the center axis, a distance between the third position and the second position in a direction parallel to the axial line is defined as a first distance (Da), and a distance between the second position and the end of the joint portion on the forward end side in the direction parallel to the axial line is defined as a second distance (Db), a difference (Db-Da) obtained by subtracting the first distance (Da) from the second distance (Db) is equal to or greater than zero mm; wherein the distance between the inner edge and the surface of the center electrode is 0.35 mm or greater. a tubular insulator (10, 10b) having an axial hole (12, 12b) extending therethrough along an axial line; a center electrode disposed at an end of the axial hole (12, 12b) on a forward end side; and a rod-shaped ground electrode forming a gap (g) between the ground electrode and a portion of the center electrode on the forward end side, wherein the center electrode includes a shaft portion, a tip portion joined to a forward end portion of the shaft portion, and a joint portion joining the shaft portion and the tip portion together; an end of the joint portion on the forward end side is located on a rear end side (D1r) in the direction of the axial line in relation to an inner edge of the insulator (10, 10b), which is a forward-end-side edge of an inner circumferential surface of a small diameter portion (10r, 14r) of the insulator (10, 10b), the small diameter portion being a part of an accommodation portion of the insulator (10, 10b) for accommodating the tip portion, which part has the smallest inner diameter in the accommodation portion; when a position separated from an edge of an end surface of the center electrode on the forward end side by 5 mm in a direction orthogonal to the axial line is defined as a first position, a position on the inner edge of the insulator (10, 10b) is defined as a second position, a position of intersection between the surface of the center electrode and a straight line (Lpr, Lprb) is defined as a third position, as viewed on a cross section including the axial line, the straight line (Lpr, Lprb) passing through the first position and being tangent, at one position, to a forward-end-side portion (R, 10p) of a contour of the insulator (10, 10b) on a side toward the first position with respect to the center axis, a distance between the third position and the second position in a direction parallel to the axial line is defined as a first distance (Da), and a distance between the second position and the end of the joint portion on the forward end side in the direction parallel to the axial line is defined as a second distance (Db), a difference (Db-Da) obtained by subtracting the first distance (Da) from the second distance (Db) is equal to or greater than zero mm; wherein the distance between the inner edge and the surface of the center electrode is 0.35 mm or greater.