An indoor unit (100, 100C) for an air-conditioning apparatus, comprising: a controller; a casing including an air inlet (1 B), an air outlet, and an air passage communicating between the air inlet and the air outlet, the air inlet being located lower than the air outlet; an indoor heat exchanger disposed inside the casing; a drain pan disposed under the indoor heat exchanger and inside the casing; an indoor fan (2, 2A, 2B) disposed at a downstream side of the indoor heat exchanger in an airflow direction and inside the casing and at an upper position of the indoor heat exchanger, and configured to supply air to the indoor heat exchanger, the indoor unit (100, 100C) being a floor-mountable indoor unit; and a filter disposed inside the casing and being located on a front side of the drain pan, characterized by the filter being obliquely installed such that the lower end portion recedes backwards from the front side of the casing; a first refrigerant leakage sensor disposed in the air passage between the air inlet and the air outlet at a position lower than the drain pan and the air outlet, and configured to detect refrigerant leakage; and said controller being configured to determine whether refrigerant leakage occurs, according to a detection result of the first refrigerant leakage sensor, wherein the casing includes a sloped portion having a sloped surface inclined upward from a front side toward a rear side and forming a part of the air passage, the sloped portion (1 E) is located lower than the indoor heat exchanger and the drain pan and is formed so as to allow the air introduced into the casing through the air inlet to smoothly flow toward the indoor heat exchanger along the sloped surface, and the first refrigerant leakage sensor is located on the sloped portion (1 E), wherein a front end of the sloped portion (1 E) is connected to a bottom face and an upper end of the sloped portion is connected to an upright portion and the upright portion is located on the rear side with respect to the indoor heat exchanger and the drain pan and extends toward a support portion from the upper end of the sloped portion. a controller; a casing including an air inlet (1 B), an air outlet, and an air passage communicating between the air inlet and the air outlet, the air inlet being located lower than the air outlet; an indoor heat exchanger disposed inside the casing; a drain pan disposed under the indoor heat exchanger and inside the casing; an indoor fan (2, 2A, 2B) disposed at a downstream side of the indoor heat exchanger in an airflow direction and inside the casing and at an upper position of the indoor heat exchanger, and configured to supply air to the indoor heat exchanger, the indoor unit (100, 100C) being a floor-mountable indoor unit; and a filter disposed inside the casing and being located on a front side of the drain pan, characterized by the filter being obliquely installed such that the lower end portion recedes backwards from the front side of the casing; a first refrigerant leakage sensor disposed in the air passage between the air inlet and the air outlet at a position lower than the drain pan and the air outlet, and configured to detect refrigerant leakage; and said controller being configured to determine whether refrigerant leakage occurs, according to a detection result of the first refrigerant leakage sensor, wherein the casing includes a sloped portion having a sloped surface inclined upward from a front side toward a rear side and forming a part of the air passage, the sloped portion (1 E) is located lower than the indoor heat exchanger and the drain pan and is formed so as to allow the air introduced into the casing through the air inlet to smoothly flow toward the indoor heat exchanger along the sloped surface, and the first refrigerant leakage sensor is located on the sloped portion (1 E), wherein a front end of the sloped portion (1 E) is connected to a bottom face and an upper end of the sloped portion is connected to an upright portion and the upright portion is located on the rear side with respect to the indoor heat exchanger and the drain pan and extends toward a support portion from the upper end of the sloped portion.