AIR CONDITIONER INDOOR UNIT

An indoor unit 2 for an air conditioner includes a heat exchanger 16 for exchanging heat between air and refrigerant; a drain pan 17 for receiving drain water which drops from the heat exchanger; and a controller for controlling a freezing operation of making frost or ice adhere to a surface of the heat exchanger. A volume of the drain pan is equal to or greater than a total adhesion amount of frost or ice to adhere to the heat exchanger during the freezing operation. Preferably, in consideration that the drain water is discharged out side of the indoor unit 2 through the drain pipe, a volume of the drain pan is equal to or greater than (the total adhesion amount of frost or ice - an amount of drainage through the drain pipe per unit time × a smaller one of an amount of time required for all the frost or ice to melt and an amount of time required for all the frost or ice to drop into the drain pan).