A spark plug comprising: an insulator which has an axial hole extending in the direction of an axis, a first inner circumferential surface which forms a forward end portion of the axial hole, a second inner circumferential surface which forms a rear end portion of the axial hole and which has a diameter greater than that of the first inner circumferential surface, and a ledge portion which connects the first inner circumferential surface and the second inner circumferential surface; a center electrode which is formed by plastic working and which has a large-diameter portion supported by the ledge portion, a projection portion projecting rearward from the large-diameter portion, and a circular columnar leg portion which is located adjacent to the large-diameter portion and projects into a space surrounded by the first inner circumferential surface; and a seal portion which fixes the center electrode within the axial hole, wherein, when C < A, A − C ≤ B − A where A represents a diameter (mm) of an imaginary cylinder having the minimum diameter required for surrounding the projection portion, B represents a maximum diameter (mm) of the large-diameter portion, and C represents an average diameter (mm) of the leg portion present in the space surrounded by the first inner circumferential surface; characterized in that when a value (L'/C) obtained by dividing an axial length L' of the projection portion along an axis by the average diameter C of the leg portion is 1 or greater, an upsetting ratio (L-L')/L×100 (%) is 13% or less, the upsetting ratio being the ratio of the difference (L-L') between a pre-formation projection portion length L and the axial length L' of the projection portion to the pre-formation projection portion length L, and the pre-formation projection portion length L being a value (V/D) obtained by dividing a volume V of the projection portion by a cross-sectional area D of the leg portion, an insulator which has an axial hole extending in the direction of an axis, a first inner circumferential surface which forms a forward end portion of the axial hole, a second inner circumferential surface which forms a rear end portion of the axial hole and which has a diameter greater than that of the first inner circumferential surface, and a ledge portion which connects the first inner circumferential surface and the second inner circumferential surface; a center electrode which is formed by plastic working and which has a large-diameter portion supported by the ledge portion, a projection portion projecting rearward from the large-diameter portion, and a circular columnar leg portion which is located adjacent to the large-diameter portion and projects into a space surrounded by the first inner circumferential surface; and a seal portion which fixes the center electrode within the axial hole, wherein, when C < A, A − C ≤ B − A where A represents a diameter (mm) of an imaginary cylinder having the minimum diameter required for surrounding the projection portion, B represents a maximum diameter (mm) of the large-diameter portion, and C represents an average diameter (mm) of the leg portion present in the space surrounded by the first inner circumferential surface;