An indoor unit for an air conditioner, comprising: a casing (1, 2, 3, 201) including a first wall, a second wall (12, 212) opposite to the first wall, a third wall connecting the first wall and the second wall (12, 212), and a fourth wall (14, 214) connecting the first wall and the second wall (12, 212) and opposite to the third wall; a centrifugal fan disposed in the casing (1, 2, 3, 201); and a heat exchanger disposed between the centrifugal fan and the first wall, third wall, and fourth wall (14, 214) of the casing (1, 2, 3, 201), wherein the heat exchanger includes a first extension part located near the third wall of the casing (1, 2, 3, 201) and extending from the first wall toward the second wall (12, 212) in the casing (1, 2, 3, 201), and a second extension part located near the fourth wall (14, 214) of the casing (1, 2, 3, 201) and extending from the first wall toward the second wall (12, 212) in the casing (1, 2, 3, 201), and the first extension part in the heat exchanger extends beyond a distal end of the second extension part in the heat exchanger toward the second wall (12, 212) of the casing (1, 2, 3, 201), characterised in that the indoor unit further comprises: a drain pump disposed between a portion of the second wall (12, 212) in the casing (1, 2, 3, 201), the portion being located near the first extension part, and the distal end of the first extension part in the heat exchanger. a casing (1, 2, 3, 201) including a first wall, a second wall (12, 212) opposite to the first wall, a third wall connecting the first wall and the second wall (12, 212), and a fourth wall (14, 214) connecting the first wall and the second wall (12, 212) and opposite to the third wall; a first wall, a second wall (12, 212) opposite to the first wall, a third wall connecting the first wall and the second wall (12, 212), and a fourth wall (14, 214) connecting the first wall and the second wall (12, 212) and opposite to the third wall; a centrifugal fan disposed in the casing (1, 2, 3, 201); and a heat exchanger disposed between the centrifugal fan and the first wall, third wall, and fourth wall (14, 214) of the casing (1, 2, 3, 201), wherein the heat exchanger includes a first extension part located near the third wall of the casing (1, 2, 3, 201) and extending from the first wall toward the second wall (12, 212) in the casing (1, 2, 3, 201), and a second extension part located near the fourth wall (14, 214) of the casing (1, 2, 3, 201) and extending from the first wall toward the second wall (12, 212) in the casing (1, 2, 3, 201), and a first extension part located near the third wall of the casing (1, 2, 3, 201) and extending from the first wall toward the second wall (12, 212) in the casing (1, 2, 3, 201), and a second extension part located near the fourth wall (14, 214) of the casing (1, 2, 3, 201) and extending from the first wall toward the second wall (12, 212) in the casing (1, 2, 3, 201), and the first extension part in the heat exchanger extends beyond a distal end of the second extension part in the heat exchanger toward the second wall (12, 212) of the casing (1, 2, 3, 201), characterised in that the indoor unit further comprises: a drain pump disposed between a portion of the second wall (12, 212) in the casing (1, 2, 3, 201), the portion being located near the first extension part, and the distal end of the first extension part in the heat exchanger.