A power supply coil for electric resistance welding for heating a steel pipe to be welded by electric resistance comprising: a metal coil that has a single-turn tubular shape and has a pair of end surfaces facing each other in a circumferential direction; a pair of metal plates that is connected to the pair of end surfaces of the metal coil; a cover that is attached to at least one of outer surfaces and inner surfaces of the metal coil and the pair of metal plates; a side wall portion that is interposed between the metal coil and the cover and between the pair of metal plates and the cover; a supply pipe that supplies cooling water to a space surrounded by the metal coil, the pair of metal plates, the cover, and the side wall portion; a discharge pipe that discharges the cooling water from the space; characterised by a first partition member that divides a first space surrounded by the metal coil, the cover, and the side wall portion into a second space that communicates with the supply pipe and a third space that communicates with the discharge pipe, wherein the first partition member has an opening that is located at a first end of the first partition member in a longitudinal direction of the first partition member and that allows the second space and the third space to communicate with each other, wherein a plurality of the first partition members are disposed substantially parallel to each other in the first space, and wherein the openings of the plurality of the first partition members are alternately formed at the first end and a second end opposite to the first end in the longitudinal direction of the first partition members from the supply pipe toward the discharge pipe. a metal coil that has a single-turn tubular shape and has a pair of end surfaces facing each other in a circumferential direction; a pair of metal plates that is connected to the pair of end surfaces of the metal coil; a cover that is attached to at least one of outer surfaces and inner surfaces of the metal coil and the pair of metal plates; a side wall portion that is interposed between the metal coil and the cover and between the pair of metal plates and the cover; a supply pipe that supplies cooling water to a space surrounded by the metal coil, the pair of metal plates, the cover, and the side wall portion; a discharge pipe that discharges the cooling water from the space; characterised by a first partition member that divides a first space surrounded by the metal coil, the cover, and the side wall portion into a second space that communicates with the supply pipe and a third space that communicates with the discharge pipe, wherein the first partition member has an opening that is located at a first end of the first partition member in a longitudinal direction of the first partition member and that allows the second space and the third space to communicate with each other, wherein a plurality of the first partition members are disposed substantially parallel to each other in the first space, and wherein the openings of the plurality of the first partition members are alternately formed at the first end and a second end opposite to the first end in the longitudinal direction of the first partition members from the supply pipe toward the discharge pipe.