A method for producing a plug that is used in a piercing-rolling mill when producing a seamless steel pipe, the method comprising: a step of preparing a plug base metal having a concave portion along a circumferential direction in an outer peripheral surface, and an arc spraying step of spraying a spraying wire rod material onto the outer peripheral surface of the plug base metal by arc spraying to form films (20a, 20b) containing Fe and Fe oxides on the outer peripheral surface of the plug base metal; wherein: the arc spraying step includes: a separating step of separating the outer peripheral surface of the plug base metal into a plurality of regions (S1, S2) along an axial direction, and a step of performing arc spraying separately in each of the regions; and in the separating step, a boundary between the regions is set at the concave portion, in the plug base metal, the outer peripheral surface is formed by connecting a plurality of divided faces (11a,..., 11f) successively in the axial direction, the plurality of divided faces (11a,..., 11f) includes two or more divided faces which are successively connected and of which outer diameters each increase progressively from a front-end side toward a rear-end side, and the concave portion is disposed at a joint between the divided faces of which outer diameters each increase progressively from a front-end side toward a rear-end side, and wherein, at the joint which is the concave portion, a slope θ1 (°) of a divided face located on a front-end side (11a - Figs. 2 and 3; 11b - Fig. 7) of the plug and a slope θ2 (°) of a divided face located on a rear-end side (11b - Figs. 2 and 3; 11c - Fig. 7) of the plug satisfy the following Formula θ 1 < θ 2 wherein the slope θ1 is an angle formed by the divided face located on a front-end side (11a - Figs. 2 and 3; 11b - Fig. 7) of the plug and a center line of the plug base metal, and the slope θ2 is an angle formed by the divided face located on a rear-end side (11b - Figs. 2 and 3; 11c - Fig. 7) of the plug and a center line of the plug base metal. a step of preparing a plug base metal having a concave portion along a circumferential direction in an outer peripheral surface, and an arc spraying step of spraying a spraying wire rod material onto the outer peripheral surface of the plug base metal by arc spraying to form films (20a, 20b) containing Fe and Fe oxides on the outer peripheral surface of the plug base metal; wherein: the arc spraying step includes: a separating step of separating the outer peripheral surface of the plug base metal into a plurality of regions (S1, S2) along an axial direction, and a step of performing arc spraying separately in each of the regions; and in the separating step, a boundary between the regions is set at the concave portion, in the plug base metal, the outer peripheral surface is formed by connecting a plurality of divided faces (11a,..., 11f) successively in the axial direction, the plurality of divided faces (11a,..., 11f) includes two or more divided faces which are successively connected and of which outer diameters each increase progressively from a front-end side toward a rear-end side, and the concave portion is disposed at a joint between the divided faces of which outer diameters each increase progressively from a front-end side toward a rear-end side, and wherein, at the joint which is the concave portion, a slope θ1 (°) of a divided face located on a front-end side (11a - Figs. 2 and 3; 11b - Fig. 7) of the plug and a slope θ2 (°) of a divided face located on a rear-end side (11b - Figs. 2 and 3; 11c - Fig. 7) of the plug satisfy the following Formula θ 1 < θ 2 wherein the slope θ1 is an angle formed by the divided face located on a front-end side (11a - Figs. 2 and 3; 11b - Fig. 7) of the plug and a center line of the plug base metal, and the slope θ2 is an angle formed by the divided face located on a rear-end side (11b - Figs. 2 and 3; 11c - Fig. 7) of the plug and a center line of the plug base metal. a separating step of separating the outer peripheral surface of the plug base metal into a plurality of regions (S1, S2) along an axial direction, and a step of performing arc spraying separately in each of the regions; and in the separating step, a boundary between the regions is set at the concave portion, in the plug base metal, the outer peripheral surface is formed by connecting a plurality of divided faces (11a,..., 11f) successively in the axial direction, the plurality of divided faces (11a,..., 11f) includes two or more divided faces which are successively connected and of which outer diameters each increase progressively from a front-end side toward a rear-end side, and the concave portion is disposed at a joint between the divided faces of which outer diameters each increase progressively from a front-end side toward a rear-end side, and wherein, at the joint which is the concave portion, a slope θ1 (°) of a divided face located on a front-end side (11a - Figs. 2 and 3; 11b - Fig. 7) of the plug and a slope θ2 (°) of a divided face located on a rear-end side (11b - Figs. 2 and 3; 11c - Fig. 7) of the plug satisfy the following Formula θ 1 < θ 2 wherein the slope θ1 is an angle formed by the divided face located on a front-end side (11a - Figs. 2 and 3; 11b - Fig. 7) of the plug and a center line of the plug base metal, and the slope θ2 is an angle formed by the divided face located on a rear-end side (11b - Figs. 2 and 3; 11c - Fig. 7) of the plug and a center line of the plug base metal.