An indoor unit of an air conditioner comprising: a main body that includes a suction port and a discharging port; a heat exchanger that cools indoor air sucked through the suction port; a cross flow fan that enables the indoor air to be sucked through the suction port, and enables cold air cooled by the heat exchanger to be discharged into a room through the discharging port; a left/right wind direction adjustment unit that adjusts left and right wind directions of the air to be discharged through the discharging port; a drainage tray unit that is disposed below the heat exchanger comprising a collection portion to collect condensate water generated by the heat exchanger; and a blade that is rotatably coupled to the drainage tray unit to open the discharging port, and whose rotary shaft is formed in an upper portion of the discharging port, the indoor unit characterized in that the drainage tray unit includes: a stabilizer unit that guides the cold air heat-exchanged by the heat exchanger towards the discharging port without backflow having a first portion spaced from the cross flow fan and separating the cross flow fan from the collection portion, a rounded second portion connecting the first portion to a third portion, the third portion being formed approximately flat towards the discharging port with a surface facing the collection portion and an opposing surface facing the left/right wind direction adjustment unit, thus separating the collection portion from the left/right wind direction adjustment unit; a wall that roundly extends from a surface of the stabilizer unit adapted to form a receiving space that receives a part of the blade when the blade is at least partially opened by a rotation of the rotation shaft, the wall separating the receiving space from the collection portion; a guide unit that protrudes towards the blade from the stabilizer unit and disposed between the receiving space and the discharge port to prevent the cold air from being discharged through a gap between the blade and the stabilizer unit in a state in which the blade is at least partially opened. a main body that includes a suction port and a discharging port; a heat exchanger that cools indoor air sucked through the suction port; a cross flow fan that enables the indoor air to be sucked through the suction port, and enables cold air cooled by the heat exchanger to be discharged into a room through the discharging port; a left/right wind direction adjustment unit that adjusts left and right wind directions of the air to be discharged through the discharging port; a drainage tray unit that is disposed below the heat exchanger comprising a collection portion to collect condensate water generated by the heat exchanger; and a blade that is rotatably coupled to the drainage tray unit to open the discharging port, and whose rotary shaft is formed in an upper portion of the discharging port, the indoor unit characterized in that the drainage tray unit includes: a stabilizer unit that guides the cold air heat-exchanged by the heat exchanger towards the discharging port without backflow having a first portion spaced from the cross flow fan and separating the cross flow fan from the collection portion, a rounded second portion connecting the first portion to a third portion, the third portion being formed approximately flat towards the discharging port with a surface facing the collection portion and an opposing surface facing the left/right wind direction adjustment unit, thus separating the collection portion from the left/right wind direction adjustment unit; a wall that roundly extends from a surface of the stabilizer unit adapted to form a receiving space that receives a part of the blade when the blade is at least partially opened by a rotation of the rotation shaft, the wall separating the receiving space from the collection portion; a guide unit that protrudes towards the blade from the stabilizer unit and disposed between the receiving space and the discharge port to prevent the cold air from being discharged through a gap between the blade and the stabilizer unit in a state in which the blade is at least partially opened. a stabilizer unit that guides the cold air heat-exchanged by the heat exchanger towards the discharging port without backflow having a first portion spaced from the cross flow fan and separating the cross flow fan from the collection portion, a rounded second portion connecting the first portion to a third portion, the third portion being formed approximately flat towards the discharging port with a surface facing the collection portion and an opposing surface facing the left/right wind direction adjustment unit, thus separating the collection portion from the left/right wind direction adjustment unit; a wall that roundly extends from a surface of the stabilizer unit adapted to form a receiving space that receives a part of the blade when the blade is at least partially opened by a rotation of the rotation shaft, the wall separating the receiving space from the collection portion; a guide unit that protrudes towards the blade from the stabilizer unit and disposed between the receiving space and the discharge port to prevent the cold air from being discharged through a gap between the blade and the stabilizer unit in a state in which the blade is at least partially opened.