Immersion nozzle

A flat immersion nozzle stabilizes the discharging flow of molten steel thereby stabilizing the molten steel surface in a mold, namely, decreasing the fluctuation thereof. In the immersion nozzle having a flat shape in which a width Wn of an inner hole is greater than a thickness Tn of the inner hole, a central protrusion portion (1) is disposed in a center section of a wall surface in a width direction of a flat section. Wp/Wn, a ratio of a length Wp of the central protrusion portion in the width direction to Wn, is 0.2 or more and 0.7 or less. The central protrusion portion (1) is disposed symmetrically as a pair; and a total length Tp in the thickness direction of the pair of the central protrusion portions is 0.15 or more and 0.75 or less of Tn.