An indoor unit for an air-conditioning apparatus, the indoor unit comprising: a box-shaped casing having an air inlet in a top surface of the casing and an air outlet in a bottom surface of the casing; an air-sending device disposed in the casing, the air-sending device being configured to suck in indoor air through the air inlet and blow conditioned air through the air outlet; a heat exchanger disposed in the casing, the heat exchanger being configured to cause the indoor air to exchange heat with refrigerant to supply the conditioned air; a vertical deflector; a first auxiliary vertical deflector; and a second auxiliary vertical deflector each rotatably arranged in the air outlet and configured to change an air flow direction in a vertical direction, during a cooling operation, the first auxiliary vertical deflector being positioned on a side of a front surface of the casing, a downstream end of the first auxiliary vertical deflector being positioned below the bottom surface of the casing, the second auxiliary vertical deflector being positioned below the first auxiliary vertical deflector, an upstream end of the second auxiliary vertical deflector being positioned above the vertical deflector, characterized in that the second auxiliary vertical deflector includes a support and a guide, the support being rotatably supported at an end, the guide being disposed at an other end of the support and protruding perpendicular to the support in side view, during the cooling operation, the guide of the second auxiliary vertical deflector being positioned below the first auxiliary vertical deflector, an upstream end of the guide being positioned above the vertical deflector. a box-shaped casing having an air inlet in a top surface of the casing and an air outlet in a bottom surface of the casing; an air-sending device disposed in the casing, the air-sending device being configured to suck in indoor air through the air inlet and blow conditioned air through the air outlet; a heat exchanger disposed in the casing, the heat exchanger being configured to cause the indoor air to exchange heat with refrigerant to supply the conditioned air; a vertical deflector; a first auxiliary vertical deflector; and a second auxiliary vertical deflector each rotatably arranged in the air outlet and configured to change an air flow direction in a vertical direction, during a cooling operation, the first auxiliary vertical deflector being positioned on a side of a front surface of the casing, a downstream end of the first auxiliary vertical deflector being positioned below the bottom surface of the casing, the second auxiliary vertical deflector being positioned below the first auxiliary vertical deflector, an upstream end of the second auxiliary vertical deflector being positioned above the vertical deflector, characterized in that the second auxiliary vertical deflector includes a support and a guide, the support being rotatably supported at an end, the guide being disposed at an other end of the support and protruding perpendicular to the support in side view, during the cooling operation, the guide of the second auxiliary vertical deflector being positioned below the first auxiliary vertical deflector, an upstream end of the guide being positioned above the vertical deflector.