An immersion nozzle comprising: 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, wherein 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, and wherein the curved line is represented by a diameter Dz in the longitudinal cross-section of the immersion nozzle in the following formula 1: Dz = H + L H + Z 1 n × Do where: L is a wall thickness of the immersion nozzle; Di is a diameter of the discharge port at the start position of the discharge port Do is a diameter of the discharge port at the end of the discharge port Z is a distance between the start position of the discharge port, and an arbitrary position apart from the start position toward the end of the discharge port; Dz is a diameter of the discharge port at the position Z in the longitudinal cross-section of the immersion nozzle; and H is represented by the following formula 2, H = L Di Do n - 1, where 1.5 ≤ n ≤ 6.0, and where 1.6 ≤ Di Do Do 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, wherein 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, and wherein the curved line is represented by a diameter Dz in the longitudinal cross-section of the immersion nozzle in the following formula 1: Dz = H + L H + Z 1 n × Do where: L is a wall thickness of the immersion nozzle; Di is a diameter of the discharge port at the start position of the discharge port Do is a diameter of the discharge port at the end of the discharge port Z is a distance between the start position of the discharge port, and an arbitrary position apart from the start position toward the end of the discharge port; Dz is a diameter of the discharge port at the position Z in the longitudinal cross-section of the immersion nozzle; and H is represented by the following formula 2, H = L Di Do n - 1, where 1.5 ≤ n ≤ 6.0, and where 1.6 ≤ Di Do Do L is a wall thickness of the immersion nozzle; Di is a diameter of the discharge port at the start position of the discharge port Do is a diameter of the discharge port at the end of the discharge port Z is a distance between the start position of the discharge port, and an arbitrary position apart from the start position toward the end of the discharge port; Dz is a diameter of the discharge port at the position Z in the longitudinal cross-section of the immersion nozzle; and H is represented by the following formula 2, H = L Di Do n - 1, where 1.5 ≤ n ≤ 6.0, and where 1.6 ≤ Di Do Do