It is intended to uniform and straighten a molten steel stream flowing out of a discharge port of an immersion nozzle, and thus suppress mold powder entrapment in the vicinity of the immersion nozzle. The immersion nozzle comprises a tubular-shaped straight nozzle body formed to extend in a vertical longitudinal direction and adapted to allow molten steel from a molten-steel inlet provided at an upper end thereof to pass downwardly therethrough, and a pair of discharge ports provided in a lower portion of the straight nozzle body in bilaterally symmetrical relation and adapted to discharge the molten steel from a lateral surface of the straight nozzle body in a lateral direction. An inner surface of each of the discharge ports has, at least in part or in its entirety, a shape defined by a curved line along which an inner bore of the discharge port in a longitudinal cross-section of the immersion nozzle passing through respective centers of the immersion nozzle and the discharge port is gradually reduced in diameter in a direction from a start position to an end of the discharge port, wherein the curved line is represented by a diameter in the longitudinal cross-section of the immersion nozzle.