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 . <EOS>