A lift-changing mechanism configured to change lift generated by a wing of an aircraft, the lift-changing mechanism comprising: - a slit that extends in a wingspan direction inside the wing and that forms openings (3A, 3B) on a lower surface of the wing and on an upper surface of the wing, respectively, the slit being a slit through which a part (α1) of an airflow (α) below the lower surface is allowed to flow toward the upper surface; and - an opening and closing member configured to open and close the slit, wherein when the slit is opened by the opening and closing member, lift generated on the wing is decreased compared with the arrangement when the slit is closed, wherein the opening and closing member is bar-shaped, passing through the wing so as to extend in the wingspan direction, and is disposed parallel to the slit, wherein the slit is opened and closed by rotating the opening and closing member around an axis while maintaining or changing a rotational direction of the opening and closing member, and wherein the axis extends in a longitudinal direction of the opening and closing member, characterised in that a side of the opening and closing member is made up of a first cylinder surface having a first radius, a second cylinder surface having a second radius, and two substantially flat surfaces present between the first cylinder surface and the second cylinder surface, the first radius being larger than the second radius, wherein, when a portion of the opening and closing member having the first radius travels to the slit, the side of the opening and closing member comes into contact with an inner wall surface of the slit, whereby the opening and closing member closes the slit, and wherein, when a portion of the opening and closing member having the second radius travels to the slit, a gap appears between the inner wall surface of the slit and the side of the opening and closing member, whereby the opening and closing member 4 opens the slit. the lift-changing mechanism comprising: - a slit that extends in a wingspan direction inside the wing and that forms openings (3A, 3B) on a lower surface of the wing and on an upper surface of the wing, respectively, the slit being a slit through which a part (α1) of an airflow (α) below the lower surface is allowed to flow toward the upper surface; and - an opening and closing member configured to open and close the slit, - a slit that extends in a wingspan direction inside the wing and that forms openings (3A, 3B) on a lower surface of the wing and on an upper surface of the wing, respectively, the slit being a slit through which a part (α1) of an airflow (α) below the lower surface is allowed to flow toward the upper surface; and - an opening and closing member configured to open and close the slit, wherein when the slit is opened by the opening and closing member, lift generated on the wing is decreased compared with the arrangement when the slit is closed, wherein the opening and closing member is bar-shaped, passing through the wing so as to extend in the wingspan direction, and is disposed parallel to the slit, wherein the slit is opened and closed by rotating the opening and closing member around an axis while maintaining or changing a rotational direction of the opening and closing member, and wherein the axis extends in a longitudinal direction of the opening and closing member, characterised in that a side of the opening and closing member is made up of a first cylinder surface having a first radius, a second cylinder surface having a second radius, and two substantially flat surfaces present between the first cylinder surface and the second cylinder surface, the first radius being larger than the second radius, wherein, when a portion of the opening and closing member having the first radius travels to the slit, the side of the opening and closing member comes into contact with an inner wall surface of the slit, whereby the opening and closing member closes the slit, and wherein, when a portion of the opening and closing member having the second radius travels to the slit, a gap appears between the inner wall surface of the slit and the side of the opening and closing member, whereby the opening and closing member 4 opens the slit.