An indoor unit for an air-conditioning apparatus, comprising: a casing, which is to be attached to a wall in a room on a back surface side of the casing; an air inlet (21, 21a, 21b) and an air outlet, which are formed in the casing; an indoor heat exchanger and an indoor fan, which are arranged in an air passage continuous from the air inlet (21, 21a, 21b) to the air outlet; an up-and-down airflow direction plate, which is provided in the air outlet so as to be turnable, and forms an air outlet passage for blowing air to be blown out through the air outlet in a region located below the air outlet, and which is configured to change a direction of the blowing air between upward and downward directions; and an up-and-down airflow direction assist plate, which is turned closer to a front surface side of the casing than the up-and-down airflow direction plate, thereby forming the air outlet passage at a position located below the air outlet, and is configured to change the direction of the blowing air in the up-and-down direction, wherein the up-and-down airflow direction assist plate comprises a plate-like portion and an arm portion wherein a rotation locus of a distal end of the up-and-down airflow direction plate crosses a locus of the up-and-down airflow direction assist plate, wherein the distal end of the up-and-down airflow direction plate includes a ridge line portion having a curved surface having a radial dimension R, characterized in that the up-and-down airflow direction assist plate includes a protruding portion protruding by a protruding dimension P on a surface with which the distal end of the up-and-down airflow direction plate is allowed to be in contact, and the protruding dimension P of the protruding portion is smaller than the radial dimension R, and the protruding portion protrudes from a back surface portion of the arm portion. a casing, which is to be attached to a wall in a room on a back surface side of the casing; an air inlet (21, 21a, 21b) and an air outlet, which are formed in the casing; an indoor heat exchanger and an indoor fan, which are arranged in an air passage continuous from the air inlet (21, 21a, 21b) to the air outlet; an up-and-down airflow direction plate, which is provided in the air outlet so as to be turnable, and forms an air outlet passage for blowing air to be blown out through the air outlet in a region located below the air outlet, and which is configured to change a direction of the blowing air between upward and downward directions; and an up-and-down airflow direction assist plate, which is turned closer to a front surface side of the casing than the up-and-down airflow direction plate, thereby forming the air outlet passage at a position located below the air outlet, and is configured to change the direction of the blowing air in the up-and-down direction, wherein the up-and-down airflow direction assist plate comprises a plate-like portion and an arm portion wherein a rotation locus of a distal end of the up-and-down airflow direction plate crosses a locus of the up-and-down airflow direction assist plate, wherein the distal end of the up-and-down airflow direction plate includes a ridge line portion having a curved surface having a radial dimension R, characterized in that the up-and-down airflow direction assist plate includes a protruding portion protruding by a protruding dimension P on a surface with which the distal end of the up-and-down airflow direction plate is allowed to be in contact, and the protruding dimension P of the protruding portion is smaller than the radial dimension R, and the protruding portion protrudes from a back surface portion of the arm portion.