A method of manufacturing a center electrode of a spark plug including: an insulator which, having an axial hole extending in an axial direction OD, has in the axial hole an in-axial-hole shoulder which reduces the diameter of the axial hole from a rear end side toward a leading end side in the axial direction OD; a metal shell disposed on the outer periphery of the insulator; and the center electrode including a large diameter portion which is inserted into the axial hole and abuts against the in-axial-hole shoulder from the axial direction OD rear end side, a barrel portion which, being smaller in diameter than the large diameter portion, is disposed closer to the axial direction OD leading end side than the large diameter portion, and small diameter portions (25 and 26) which, being disposed closer to the leading end side than the barrel portion, are smaller in diameter than the barrel portion, the method comprising: first step (A to D) of preparing a cylindrical electrode member as the material of the center electrode; second step (E and F) of forming a medium diameter portion larger in diameter than the small diameter portions (25 and 26), from the leading end to rear end of the electrode member, using an extrusion; a third step of forming the small diameter portions (25 and 26) on the leading end side of the medium diameter portion using an extrusion after the second step (E and F); and a fourth step of, when the cross-sectional area of a cross section of the medium diameter portion perpendicular to the axial direction OD is taken to be S1, and the cross-sectional area of a cross section of each small diameter portion perpendicular to the axial direction is taken to be S2, forming the barrel portion by extruding the medium diameter portion after the third step when the value of ((S1-S2)/S1×100) is 30 or more. an insulator which, having an axial hole extending in an axial direction OD, has in the axial hole an in-axial-hole shoulder which reduces the diameter of the axial hole from a rear end side toward a leading end side in the axial direction OD; a metal shell disposed on the outer periphery of the insulator; and the center electrode including a large diameter portion which is inserted into the axial hole and abuts against the in-axial-hole shoulder from the axial direction OD rear end side, a barrel portion which, being smaller in diameter than the large diameter portion, is disposed closer to the axial direction OD leading end side than the large diameter portion, and small diameter portions (25 and 26) which, being disposed closer to the leading end side than the barrel portion, are smaller in diameter than the barrel portion, the method comprising: first step (A to D) of preparing a cylindrical electrode member as the material of the center electrode; second step (E and F) of forming a medium diameter portion larger in diameter than the small diameter portions (25 and 26), from the leading end to rear end of the electrode member, using an extrusion; a third step of forming the small diameter portions (25 and 26) on the leading end side of the medium diameter portion using an extrusion after the second step (E and F); and a fourth step of, when the cross-sectional area of a cross section of the medium diameter portion perpendicular to the axial direction OD is taken to be S1, and the cross-sectional area of a cross section of each small diameter portion perpendicular to the axial direction is taken to be S2, forming the barrel portion by extruding the medium diameter portion after the third step when the value of ((S1-S2)/S1×100) is 30 or more. first step (A to D) of preparing a cylindrical electrode member as the material of the center electrode; second step (E and F) of forming a medium diameter portion larger in diameter than the small diameter portions (25 and 26), from the leading end to rear end of the electrode member, using an extrusion; a third step of forming the small diameter portions (25 and 26) on the leading end side of the medium diameter portion using an extrusion after the second step (E and F); and a fourth step of, when the cross-sectional area of a cross section of the medium diameter portion perpendicular to the axial direction OD is taken to be S1, and the cross-sectional area of a cross section of each small diameter portion perpendicular to the axial direction is taken to be S2, forming the barrel portion by extruding the medium diameter portion after the third step when the value of ((S1-S2)/S1×100) is 30 or more.