A mold that solidifies molten steel injected to an inner space thereof, comprising: a body having the inner space; and a convex member that protrudes in a direction from an inner surface of the body to the inner space, wherein a protruding length of the convex member in a direction from the inner surface to the inner space gradually decreases, wherein the body comprises: one pair of long side members each extending in one direction and installed to face each other in a direction crossing the extended direction; and one pair of short side members extending to cross the long side members, respectively, and installed to face each other, thereby sealing a portion between the one pair of long side members, the convex member is formed protruding from the short side members to the inner space of the mold and extends downward from an upper portion, wherein the upper portion of the convex member is positioned at the same height as an upper portion of the body, and wherein the vertical extension length (H 2) of the convex member is less than 0.5 times of the vertical extension length (H 1) of the body, and a variation rate is adjusted so that a decreasing rate in which the inner surface extension length of a short side part decreases as the protruding length of the convex member gradually decreases in a downward direction is equal to, synchronized with, or corresponds to a contraction rate in which a length in a Y-axis direction of a short side solidification shell gradually decreases in the downward direction as the short side solidification shell is contracted. a body having the inner space; and a convex member that protrudes in a direction from an inner surface of the body to the inner space, wherein a protruding length of the convex member in a direction from the inner surface to the inner space gradually decreases, wherein the body comprises: one pair of long side members each extending in one direction and installed to face each other in a direction crossing the extended direction; and one pair of short side members extending to cross the long side members, respectively, and installed to face each other, thereby sealing a portion between the one pair of long side members, the convex member is formed protruding from the short side members to the inner space of the mold and extends downward from an upper portion, wherein the upper portion of the convex member is positioned at the same height as an upper portion of the body, and wherein the vertical extension length (H 2) of the convex member is less than 0.5 times of the vertical extension length (H 1) of the body, and a variation rate is adjusted so that a decreasing rate in which the inner surface extension length of a short side part decreases as the protruding length of the convex member gradually decreases in a downward direction is equal to, synchronized with, or corresponds to a contraction rate in which a length in a Y-axis direction of a short side solidification shell gradually decreases in the downward direction as the short side solidification shell is contracted. one pair of long side members each extending in one direction and installed to face each other in a direction crossing the extended direction; and one pair of short side members extending to cross the long side members, respectively, and installed to face each other, thereby sealing a portion between the one pair of long side members, the convex member is formed protruding from the short side members to the inner space of the mold and extends downward from an upper portion, wherein the upper portion of the convex member is positioned at the same height as an upper portion of the body, and wherein the vertical extension length (H 2) of the convex member is less than 0.5 times of the vertical extension length (H 1) of the body, and a variation rate is adjusted so that a decreasing rate in which the inner surface extension length of a short side part decreases as the protruding length of the convex member gradually decreases in a downward direction is equal to, synchronized with, or corresponds to a contraction rate in which a length in a Y-axis direction of a short side solidification shell gradually decreases in the downward direction as the short side solidification shell is contracted.